Im a 75yo BM and SM of 5.  With the ages your soon-to-be SKs are, they should be about "done", or launched as we say on Steptalk, especially the 22yo and 20yo girls.  Are they in college? If Mom has done her job, all you really have to do is be polite and civil.  Danger signs would be girls still living at home, not in school or working, drugs, other addictions, etc.  Im guessing the 17yo noy is still home and in school.  Same thing, be polite and civil.  Leave all discipline to Mom, it doesnt work for stepparents to do it.

When and if problems arise, just read around on Steptalk, I promise others have gone thru the same or worse.  There is a wealth of wisdom on here.  Good luck!

Not a SD, but a SM and a SK whose dad has had a couple long-term relationships over the years.

These "kids" in your life are already grown, so your presence or acceptance as a father-figure is likely to not happen. As an adult, what I want for my parents is for them to be happy. I want them to date people that will make them happy and treat them well.

My mom has been remarried for over 15 years now, but my dad has had three significant GFs in that time. My SD is parental to me because he was around when I was a minor. My dad's SO now? She will never be viewed as a "mom" to me, but I have care and fondness for her. She has made my dad happy (in his own way), and she has always been a pleasant person. I really like her and am very thankful for her in my dad's life. If it weren't for COVID, I wanted to spend some time with her this summer one-on-one to get to know her better and do a few things that we have mutual interests in. 

Now, regarding your feelings of parenthood, are you saying you now wish or want to become a parent? If so, have you talked to your SO about that? Are you two at ages where it's possible and wanted? Are there other avenues that you can express that want for parenthood (e.g. becoming a foster parent or foster-adopting; being a Big Brother)?

I get it. I wanted kids before getting married, but also thought that having SKs would be enough if it didn't happen. It hasn't been enough. If anything, it has pushed me more to want to have kids. So, here we are, spending a lot of money and time trying to make that happen.

I think lowering the expectations you put on yourself to try and be a dad will help a lot. Be kind, be good to their mother, and let her parent them. Unless they are total a-holes (and if they are, we can have a different conversation), they should respond much better to you as a good man than trying to be a replacement or "bonus" dad.

And if you want your own kids, start plotting out what that looks like. If it's not possible, know that it's okay to mourn losing something, even if you didn't think you wanted it.

These kids are grown - don't think of yourself as "stepdad" - think of yourself as "Mom's husband", and try to build a respectful relationship with them. Don't try to parent, leave that to their mother.  Just be kind and caring and see what grows from there. They may never see you as "stepdad", and that's okay - you can still have a very good relationship with them.

At that age they are who they are. Hopefully you can find things you like about them or common interests. Similar to working with someone that is much younger than you.

Concerning how you relate to your new step-children at their ages, that varies so much between individuals that you will need to feel your way. Given you know their mother well enough to be marrying her, presumably you will have met the kids plentry of times and got to know then well enough to have some idea how they see things.

My biggest mistake when I acquired my daughter, already in her 20's, was thinking that at her age parenting did not come into it. I envisaged a relationship similar to that I had with my new sisters-in-law: close, familial and adult, and she had similiar ideas. Time showed that we were wrong. She had what I suppose we could call a 'fathering deficit', and my being privvy to all of my wife's distinctly parental thoughts and feelings about her meant I could hardly have avoided thinking of her the same way. We developed a relationship with a distinct father-and-adult-daughter flavour, and as long as she is happy that I refer to her as my daughter I will go on doing so.

You should not be the one worrying about fitting in. Your adult Skids are the ones who have to fit in to their mother's marriage to  you.

These kids are raised, with the exception of a couple of more years of seasoning on the 17yo.t ta 

You and your bride have to be absolutely clear with each other that each other and the marriage are the sole priority for each of you.  Above all and everyone else.  Including any kids that are in the mix.

Minor children are the top adult relationship responsibility, but never the priority.  There are no minor children in your blended family marriage, at least there won't be  in less than a year.  Do not let the adult Skids interfere in you rmarriage.  Their mom needs to be on board with  you on this.

Good luck.

I've been childless by choice, then I fell in love with a guy with 3 kids. So here I am. 
Unlike you, if anything this experience has proven to me I was right all along, not to have kids.

My Skids were adopted from the foster system. So when they're acting up, I have a hard time gauging if it's regular kid stuff, a lack of discipline, or their messed up genetics. Makes it stressful.

Welcome.

You've come into it late in the game, which is hopefully a good thing. Hopefully you may only have a few years before they leave the nest. Are they decent human beings?

You don't need to be a SD at all. Stay the hell out of everything, it's nothing to do with you, especially at the age they are. As long as they are respectful to you, I would do my best to stay out of everything that happens as long as they don't cause you stress and effect your life. Do not get involved in any way shape or form. If you don't live with your partner then I suggest you don't for the time being.

You need to make it very clear what you are willing and not willing to do. E.g lifts back and forth to places, cooking for them, telling them to tidy up e.t.c

I reitereate. Don't even try and be a dad, just be a decent person to them and show that you don't really care and that everything even the discipline is left to the mum. Do not get involved, and for the time being until you have spent enough time there to see how they are, how they treat the mum, the relationship with the dad and all those things, do not move in with her

I can't be clearer

Like you, I have no BKs... by choice.  My DW and I have been equity life partners in all things including raising her son as our top relationship responsibility.  Though it is our relationship and each other that have always been our joint priority

He was 15mos old when we met. We maried the week before he turned 2yo.  We have never lived nearer than about 1200 miles to SpermLand and the SpermClan.   So our blended family situation was about as uncomplicated as it gets.

want to see their mother treated with respect and dignity.  Give them that and they will grow fond of you.  If you can master dealing with them respectfully, kind and with a level head, all will be fine.  You cannot force it.  But mostly at thier age, be the husband their mother deserves.