You're not selfish at all. Having the kids fulltime will be very different. And they will be grieving a huge loss. You're right to think it through and to push your fiance for communication. 

I'd advise putting the engagement on hold. Get a yours only place and move out. This is going to be hell on wheels, and you're incredibly lucky it didn't happen after you were already married. That family is going to need to grieve without the complication of becoming a stepfamily. And your DF needs to learn how to parent ON HIS OWN. Men naturally expect the woman to take on the nurturing and family management role, and these kids are not going to want it from you. 

It is a huge red flag that your SO is not making plans now. You need to have a "come to Jesus" talk with him and if he doesn't give you the information you need, then you need to take a huge step back. Put the wedding on hold and do what NTM suggested, get a place of your own and shift back to "dating" mode. DH and his children will need to figure out their new normal, and then you can tell when and if it will work for you.

Steplife is full of surprises and I experienced this one, too.  When DH and I got together, I realized we'd have his 3 kids for visitation tho I didn't realize how frequently.  I had 2 of my own full time so that meant 5 kids with 6 years from oldest to youngest every weekend and all summer.  It was a rocky time full of adjustments for everyone, I now realize.

Long story short, the SKs began to move in, one at a time, over a 9-month period.  BM had a new bf and the kids weren't welcome in their life tho no one put it that way.  Thankfully, I had started therapy right before OSS moved in first and that helped tremendously.  He wasn't a problem but when SD came 3 months later, that was more challenging.  YSS arrived 6 months later.

To my surprise, having them full time was easier than visitation because the transition times didn't exist.  Therapy was a Godsend for me, you might want to consider it.

There's no question that this changes your life.  You're the only one who can decide if being with your fiance is worth it.  Honestly, I don't know if I'd do it again but I had 2 bios and didn't want to put them thru another upheaval after a terrible divorce.

You're not horrible for having hesitations.  Do you work?  If you needed to be a SAHM for awhile, could you guys afford it?  These kids are going to be going thru some trauma and may need someone there for awhile.  Do you want to have your own bios?  Is your fiance a good dad and not a Disney dad?  You have a number of things to work out, that's why I advise counseling.

If you decide to stay, you and your fiance are going to have to have ongoing serious conversations.  You'll both need to be on the same page.  This will be a pivotal experience in the kids' lives.  A lot is riding on your decision 

Good luck.

Just looking through your post - you say 1) the thought of having his children full time terrifies you 2) you don't know if you are cut out for it and 3) your internal voice is telling you to get out while you can.  

All this adds up to the fact that you have serious doubts about future living arrangements.  It would be so difficult for the kids if you move out AFTER they have moved in with their dad, having already lost their mother.  I agree with the post above, by ntm, this family need to grieve on their own and your fiance needs to learn to be a full time parent - not to shove it onto you.  I suggest your own place for now and a part time relationship with him, as he will need all his emotional energies for his kids. 

The kids are going to be there 24/7/365. No going out to ear by yourselfs.  Or doing anything with out the kids.  Those kids need real help. With the losing of there mother, this help should start now.  Your SO isn't helping his kids now his head is in the sand 

You are not selfish at all! This is a HUGE change and something that you had no voice in. It would be different if you and he decided to fight BM for full time- this is just happening to you. 

While your fiance doesn't have a choice you do. 

Personally, I wouldn't want to compete with a ghost for the rest of my life and no matter what I did, it would never be as good as "my mom would have done it". That amount of pressure is just too much for me. Especially since you aren't married yet. 

This is your life too. You are allowed to make active decisions in how your life will turn out. 

No sugar coating from me - girl, run like your life depends on it (because it does).

Seriously, he won't make any plans or even talk about it? Right, cuz "it will all work out in the end" or some other fluff bs? Mmmhmm the "working out in the end" will be him expecting you to do the most. The most everything, because, "you should love them like your own because you love me".  

Move out now - as above poster stated - it will be harder for the children if you do it after they move in after their mother dies. Probably easier on you as well - you shouldn't feel any guilt over this - but you will if there are crying children as you are walking out the door. 

Date if you want and see how things go after he becomes a full time sole parent, but my guess is it will be a long time (if ever) before things are "normal" and you might consider moving back in.

Sorry for what probably feels like the loss of your planned future.

I personally would leave now. This doesn't sound like a situation you want or are prepared for. And it doesn't sound like your fiance is being very proactive in dealing with things. Leaving after BM dies will be more difficult for everyone concerned, and it will not be a good look.  This is the time to consider yourself and your own goals and desires,  because no one else is going to do that.  

What is on the horizon is a huge change.. and it will be one with a lot of pain for everyone involved.  Of course.. technically all SO of partners with minor children are one "car crash" away from having full custody.  But.. you are facing a more certain reality.

You may want to seek some counseling for yourself to work through your feelings.. to understand whether you truly can be up for this challenge.

Your SO needs to be more on the ball of what his expectations are.

Also.. are there other family involved in the kid's lives.. like their maternal grandparents? it's possible they may want to take more time with the kids? who knows.

If you want kids of your own.. you need to consider the size of your home.. resources etc.. how will it really work

But.. if you truly don't see steplife full time as a good reality.. better to cut losses 

Run fast and run far and do it now. Really think about the dynamics of how the household will be when your SO feels guilty that his kids' BM passed. You already say that you look forward to the SKs leaving and going back to BM's. Why is that? I mean really think about it. Are they already Disney parented? Does your SO have loose parameters around his parenting of them? If that's the case, imagine how bad it'll get when he magnifies any guilt or fear due to the SKs losing their BM. Any annoyances resulting from your SO's parenting or lack thereof will exponentially get worse. You won't get that break either. If you dare suggest that the kids go to Grandma's or something so you and SO can have a date night, you'll be painted as the evil selfish SM for having the audacity of wanting to nurture your adult relationship. You will not be "allowed" to have needs as a spouse. You will not be "allowed" to establish new traditions as the woman of the house. BM will remain the woman of the house. You will come a distant last place to the SKs which is common in divorced families, but will be expoentially magnified when your SO feels super guilty because of his kids having lost their BM. That is not to say how tragic it is for them, but if you really think about how the new dynamics will go, I would personally get out ASAP. It is not selfish to think about how it will be. You are not selfish or unreasonable and don't let anyone tell you that you are. This would be an untenable situation for me. You won't ever get a break from the kids and you won't be "allowed" to have needs as a spouse/wife. Everything will take a back seat to the kids' grief. It is not selfish to choose you and decide that this is not the life you want. You do not have to sentence yourself to a lifetime of a family grieving or to being a distant last place to another woman's ghost. Do not feel any guilt whatsoever. It is healthy to realize that this isn't the life for you. 

If you get out now (keep dating him if you want, but from your new home), you can observe how things are progressing and dip a toe back into the water when you think that things are as near to normal as you can expect. If you don't leave now, you won't get that chance again ... 

Don't be sucked under and drown before you decide to claim your life back.

Get Your Own Place.

The grieving and then your role and the massive changes. If you have an intuitive voice speaking to you now, its in your best interest to listen to it.

Love is not enough.

Losing BM by end of the year is only five months away. That your fiance is in the dark about what is happening is terrible. He is the one who will have to care for his grieving children. I take it theirs is a strained relationship? Can he pull some compassion together and have a conversation with her about it though? They should be making plans for their kids.

As for you, if your inner voice is telling you to get out now while you can, pay attention, and you are not a horrible person for wanting to do so. I agree with JRI that individual therapy might help in this case. It will help you figure out what is really important for your own happiness and wellbeing.

If that BM didn't care enough to put some life insurance in place to make sure her kids are taken care of when she passes, I'd ask her to set up a GoFundMe for posthumous child support and if she refuses then let that man do 100% of the physical/financial provisions for his kids. 

1) Are you sure BM is going to pass? Without any details, the story sounds a little sketch. Even your SO doesn't know the details? Who is taking care of the kids the other half of the time right now if BM is 5 months from dying? Where will they be after she passes? This story is full of holes and the fact that your DH doesn't seem to have any info is suspicious.

BUT-

2) There is already something wrong with this situation, even if BM is healthy. You can't wait for the kids to go back. Why is that? Do they invade adult space (come in the bedroom or monopolize the living room or kitchen?) Do they interrupt or demand constant attention and entertainment or can they play independently? Do they have a bedtime routine that leaves adult time after they are in bed? Are they respectful? Do they clean up after themselves or if too young, does their dad? How does your partner parent? Does he change toward you when they are around? Does he share your values when it comes to parenting? Even spending 50% of your time miserable sucks. 

3) Do you want kids? If not, even 50% of the time with young kids sounds crappy. If so, how will that work with 2 other kids? Do you want a house full of kids? You are in your 30s. It's time to decide.

You are absolutely not a bad person for feeling the way you do. 

Get your own place (if you're not ready to call it off) and see how he parents his kids during this time. This can determine your future if you really love him.

When I met SO his kids were 8,11,12. They went to BMs EOWE (6 days a month) with excuses through out to not take them. That lasted two years then 24/7 after court in 2018.

Had I known I would have kept my apartment and not moved in with him when I did. SO isn't a bad parent (kind of lazy IMO) but it's been ALOT. Granted our "life" is pretty normal... I have basically zero control on how the house functions. Even after all these years everything I say he takes it in defense mode.  I have a good relationship with my SS18 and SD16... SS20 I don't and I can't stand living with him to the point I've almost moved out. We don't speak, no happy birthdays, not even hi's. I've done soo much for them but he is just like his BM. No saving. Nature vs nurture and nature took over this one. 

Even living with them you have no control on how they turn out. 

I'll add for reference I'm 37 and bioless. SO is 46. 

Your SO is selfish for not considering and discussing the likely midterm demise of his failed family breeding partner and becoming the full-time parent to his failed family progeny. That he is not considering the impact of that likely outcome on your life... makes him a STBX IMHO.

If he engages, discusses, and plans with you on your home/relationship becoming the full time home for his children, even then you may not want to take that on.  If you choose not to, even then you are not being selfish.

It is about living our best lives. If that cannot happen when life is invaded by a prior breeder mate's invading progeny, that is on them. Not on us.  If they cannot effectively parent and keep their spawn under control, and will not tolerate us to do it, they are not worthy to be our mate.

My DW and I met when SS-30 was 15mos old. We married the week before he turned 2yo.  This week is our 29th anniversary. His 31ist B-day is next week.   We are proud of the man we raised together.  He is a successful viable adult, is a man of character, honor, and standing in his profession and community.  Sadly, his three younger Spermidiot spawned half sibs are not. Spermidiot spawn #2 is on the dole, #3 is in prison, and #4 is not far behind his inmate immediately elder brother.

IMHO being a SParent is the hardest role in the parenting world.  No doubt, many will agree with me.  We have no official authority, no legal standing, often we have a failed adult, parent, and mate as our partner, and it is not unlikely that our SO’s X is a toxic POS manipulator PASing evil influence on our Skids and on our mate.  As well as, as often as not, our SO expects us to fill the roll of their X in the raising of their failed family progeny

My situation is comparatively rare.  My DW and I agreed from early in our relationship that as equity life partners we would be equity parents to any children in our relationship. As it turned out, SS is an only child in our marriage.  We did have toxic opposition to deal with in the SpermClan. Even with the likely pending demise of the BM in your situation, there are toxic risks. The risk of a worshipped dead bio parent, BM's family being toxically involved, your own SO’s not unlikely guilt dover ending his marriage, the death of the mother of his children (if it comes to pass) etc...

Also, for reference, I am 59 and a non-breeder. DW will be 48 in a month and was a single teen mom.  We were both at university when we met when I was 29, and she was 18.

Edit: In addition... if my partner was not a viable confident adult parenting with standards of behavior and standards of performance for those children, while engaging in a life together as equity partners (including full communication and respect for me input), and equity parents to any children in our marriage regardless of kid biology, I would not be in that relationship.

Listen to the nagging voice in the back of your head saying get out while you can. I wish someone had told me to run the other direction before getting married. My husband had 50/50 when we were dating (which I was okay with) and now that we are newly married, we learned his 4 year old: has autism, may have ADHD, can't potty train, still smears his poo, and has behaviour issues. The cherry on top? The bio mom and my husband signed papers to give us full custody next year.  I WISH someone had warned me before I locked myself in. It just gets worse. What would I do if I were you, you ask. You need to run the other way! And no, you are not horrible for thinking about yourself and feeling like this isn't the life you signed up for. I have said the exact same thing: "This is not what I signed up for." There's going to be so much "Guilty Dad Syndrome" parenting happening since the kids will have lost their mom and your fiance will be focused on helping them grieve and then guilt-parenting them in the future. You will not be a priority. The nagging voice is your intuition and a red-flag warning to you. ALWAYS listen to it!

I am 34 and met my husband when I was 32 and wish I would have waited it out for someone without kids.