First I want to say your feelings are valid so don't feel shame for having them. Second, take a moment to ask yourself about the character of your partner.

This man knew he had the possibility of a child and refused to get answers until she is almost an adult then does not regard how you will feel with having this dumped on your lap without so much as an "are you okay with this?"

Your feelings are totally valid.  I can't think of many people that'd be happy to have a 16 year old with whom neither they nor their partner had a relationship start spending weekends.  Especially when there's likely to be a lot of resentment on the part of the teen and guilt on the part of your partner involved.  I'd also be incredibly annoyed and wondering Why Now?  He's known about this kid for years and did nothing, so why is he determined to destroy the status quo now?  

Also, why in the world would he want to search out this child and arrange to spend weekends with her when he's not avaiilable on the weekends?  Why would his presumed child want to spend her weekends with her father's girlfriend instead of her mother or her friends?  This just seems like a bad idea all around.  If he's waited this long, maybe he should wait another couple years until she's an adult and can make her own decisions regarding a relationship with him.  I think all you can do is let him know that you will support him having a relationship with her as long as HE is the one putting in the effort, meaning that she doesn't come to your home unless he is there.  At her age, I'd think he'd do better to arrange to meet her for lunch or dinner or take her to do something outside of the house to get to know her rather than expecting her to do visitation for weekends.

NO. You are not babysitting his DD.  If he wants to do visitation with his DD its all on him. He with her 24 hours a day 

He goes anywhere DD goes wirh him.   You make arrangements to be going out on those weekends by YOURSELVE

Your feelings are normal. Let him know that while you understand he wants to try to connect with her, you are not prepared at this time to be part of it. It's kind of odd that he thinks he's going to look up a teenage daughter (that he abandoned), find her, and she's going to want to spend weekends in your home right away.  She may not even want to meet him, much less come to your home. He's putting the cart way before the horse, so I wouldn't worry too much about weekends right now.  Let him seek her out and see where she stands - it it comes to the point of her coming to your home, you can set more boundaries then.

The absolute bottom line is that if he isn't there then she isn't there. She is not your daughter and she won't be visiting you, she'd be coming for her father. You absolutely should NOT be doing any of the work. Please make that clear to him and do NOT feel guilty about it. He wants a relationship with his daughter then let HIM work on it. You should simply avoid putting a spoke in his wheel. That's all. He should have no expectations of you - it will be hard enough accepting a total stranger into your home. Has he even met her??!

Your feelings are very valid.  I can't help to think of those stories I heard about dad/daughter re-uniting once they were teends/young adults and it turning VERY creepy and incestuos-it's happened.  Because they weren't 'raised' together and yet have that biological connection.  It's a real thing.

I would specify NOW you are not open to weekends, he can have a relationship w/her outside or his place for now until something is built and then if she wants the occasional overnight, it will ONLY be on his days off.  He has NO right to not be a part of her life 16 years and then NOW throw this at you- you married him not knowing he even had a kid-he intentionally hid it from you.  He lacks some serious character for sure.  What kind of guy does this?  

Why would you feel guilty or beat yourself up over not wanting to date a married man or a man with children?  I applaud your clarity, confidence, and maturity in questioning your SO about being married and having children at the very beginning of your relationship.  Good for you. Stop the guilt and beating yourself up crap like what you did was wrong.  You did nothing wrong. Your idiot SO on the other hand.......

You and this POS have not built a life together. He has extorted you out of 10 years of your life, used you, victimized you, and he continues to commit these unforgivable victimizations of you for every second you tolerate him in your life.  You have to respect yourself more than this POS does. He does not respect you at all. You are his sugar mama, bed warmer, mark and nothing more.  No one does what this POS has done to you to anyone that they love and respect.  If you learn nothing else from this wasted ten years, make no mistake about that.

You may have matured past wanting children but you can never mature past the unequivocal fact that your SO is a lying POS who has strung you along in a relationship based on pure and complete bullshit regarding him having a child. He knew. He was married to his daughters mother. 
You are in all likelihood right about the 15yo's BM telling her that your SO abandoned her. Guess what.... HE DID!  What a winner he is. 

Why bother sending money for a kid that he has never supported, never recognized, never seen,  and whose BM never held the SpermDonor accountable for supporting? Not your kid, not your lie, not your problem and if I were you I would for damned sure not allow one cent of relationship funds to go to guilt payments made by your idiot POS hopefully STBXBF.

I smell a college fund grab by BM and a guilt ridden pay off for his characterless and disgusting abandonment of his daughter for  15 YEARS!!!! by your POS BF.

You can take a POS and polish it incessantly and at the end of that effort, all you have is a shiny POS.  Quit wasting your time, life, and youth polishing the complete and total POS that you have wasted 9 years of your life on.

Take it all, put his ass in the gutter, get on with your life, and do not waste one second on guilt or regret over purging the shit that he is from your life.  The 15yo is certainly a victim of your POS BF though no more than you are. In many ways.. she is far less his victim.  She at least has not been suffering the stench of his POS presence for the past 9 years.

Shred him, destroy him, leave him quivering in the homeless camp under the local overpass and go live your best life.

Living well is the best revenge on idiot, lying, disgusting POS people who would suck the life out of you for a decade with their complete lack of character and lying bullshit.

No matter how in love a person is with a POS, no matter how long you spend polishing that POS... at best all they will ever be is a shiny piece of shit and your hands will smell like the shit that he is until you pull  your head out of your ass, gain clarity, and purge the shit from your life.

He wants his long abandoned guilt teen in your home for you to supervise while he is at work... Nope. Next time he leaves for work call a locksmith to re-key the locks, have your lawyer nail his ass with a cease and desist order, cash out every asset, and show him how you deal with a lying POS who pulls what he has pulled with you.  You may not be able to build a wall around your home but you can do the next best thing by booting his ass out, nailing him with a killer shark attorney who you will keep at his throat until he fades away, and by hiring a security firm to post an armed guard at your home to shoot his as if he even thinks about showing his POS face anywhere near you again.  Start broadcasting his lying child abandoning bullshit wholesale throughout your friends and family network making sure that his employer and coworkers, and particularly his parents and family, will catch wind of it though the grapevine.  Everyone he could possibly ever run into should know what he is.  Not that it will be all that hard to figure out from the stench that undoubtedly wafts from his shit stained life.

End this POS... permanently and enjoy launching into your new life adventure with this POS and his is secret failed family baggage fading into your past.... at a high rate of speed.

Make sure you rub his nose so firmly in his shit that he will never get his own stench out of his sinuses.

And have fun doing it.

Living well is the best revenge. Go live your best life... and enjoy making that your best revenge.

Good luck. Take care of you.

Did I mention that your BF is a lying POS?

Rags is a stepdad/Adoptive Father to his stepson. He has a lot of wisdom and tells you like it is. I would go full inferno on your DH over his lies. Life is too short to waste on a lying POS.

I don't blame you for not wanting to be involved in the mess your SO  made years ago and is just now getting around to dealing with. 

He came into the relationship on a lie.. a pretty big lie.  It's unlikely that he had zero idea his EX had a child.. unless it was truly a "I was in the army in germany.. married a girl but we divorced when I went stateside and I had no idea she was pregnant at the time" situation.  It's unlikely that he wouldn't have heard through the grapevine that his recently EX was growing a child.

So, one of the foundations of your relationship homes was made with quicksand vs concrete.  He lied.. and he lied about a pretty big and potentially life altering situation.. having a child.. not only that.. but it tells you he is the kind of man who is happy to turn his back on doing the right thing.. when the right thing is tough to do.

But... at year 5.5, you do kind of pick sides in this when you are happy to sweep this "child" under the rug even though you are fairly certain the child is his.  I will say, this doesn't really give you a lot of moral high ground.. while this was HIS child (allegedly).. you were ok with him continuing to shirk his moral and financial and otherwise responsitility.  So, you DID stay with a guy even though he told you a pretty big lie and one that shows him to be not particularly upstanding morally.  I would have a hard time maintaining respect for him at this point.. but you did decide to stay with him.. maybe this one issue was an apparent blindspot in an otherwise honest and respectable person?

Don't get me wrong.. even though you did choose to put your head in the sand at that time.. doesn't mean that you should automatically have to pay the penance of accepting this situation and babysitting his child.

Now.. it's several years down the line.  While I can understand that you didn't originally want to date a man with kids.. you have known for many years that he likely does have a child.  I think your reasons for not wanting him to pursue a relationship are not quite as altruistic as not wanting to upturn his daughter's applecart of a life.  Pictures and social media are decieving.. she may well have some serious sadness about not knowing or having a father in her life.  It is speculation what her mom has told her.  It sounds like it may be likely that mom believes he did know she was pregnant and chose to avoid contact to avoid responsibility.  He did abandon his child.. and that is a fairly big ethical failure. 

So, what should he do?  Honestly, if there is a likelihood this child IS his.. he should contact the girl's mother and see if it would be possible to officially figure that out.. dna test.. and he should be prepared to step up and start supporting.. perhaps even back support.  

Now, I know that isn't what you signed up for.. but this guy lied to you.. BIG TIME.  I would be very angry at him.. but that anger would have started at the point where that "maybe" child appeared.. from that point forward.. her existence was a proverbial shoe that has now come to a point where it could drop.

Is he being selfish for pursuing the relationship now? Will it negatively impact you and him financially (it shouldn't impact you.. but not sure what this would mean for his ability to contribute to your joint finances).  

I can say that without a doubt, this situation would be super difficult to accept.  But, if he has to seek this contact with her.. he should go through her mother.. and if mom rebuffs him now.. he will have to wait until she is an adult and try directly... He needs to understand that his hiding for 15 some years means that he is definitely not in the position to demand a relationship with anyone.

Kids were your deal breaker and you bared your soul being honest about no children.  It took 5.5 years before his ex told him he had a child? Did he start paying cs? Sounds fishy.

He completely disregarded your very important deal breaker.  How selfish! If he lied to you about something so HUGE as this, doesn't it make you wonder what else he lies about?

If not having kids is still your deal breaker, then leave him. Be true to yourself or you will lose yourself. 

There are so many unknowns...what bm is like, what this kid is like, etc. Perceived or actual parental abandonment is one of the most painful childhood traumas. This child WILL have issues. And he talks of just letting her spend the night already, without knowing her? So he has been talking to his ex? Do you know n talk to his ex?

From my experience guilty dads pay through the nose for the rest of their lives. BM will take advantage. The kids ...and bm possibly...will soak you financially,  emotionally,  etc. BM could sue for unpaid cs over the years...

You told him what your deal breakers were. Decide if this is still a deal breaker and either leave him completely, move out n date him eo weekend,  or risk bitterness at giving up on something that was a crucial deal breaker for you....plus the potential tidal wave of hell that can come with a bitter BM...or worse, she wants him back...and a child who will automatically have issues.

If I was the BM, I am not sure I would trust an ex like that or his SO...no offense, just that BM will be automatically against you probably. 

Sorry to sound bleak. You are young enough to cut him off n live a good life. 10 years from now....you may be battered from all sides (BM hating you, SD treating you like crap, and your SO blaming you for trying to discipline his kid while you care fir her while he isn't there. Etc.) And in 10 years, sd could still be living with you...the BM still in your life etc.

Listen to your gut. Get more info if you need to such as meet BM, find out if sd has been in counsel for abandonment, etc. See how much work it is already? If you do not want kids, move out and maybe consider moving on.

He lied to you about something you were very serious about. Doesn't sound like he'd have your back with step kid issues.

Best to you. Be happy...life is SHORT!!

I hope Little Bird has read all the wise words written here and is busy making changes to her life accordingly. 

But...I've also seen newbies get solid advice they don't want to hear, and disappear forever. 

WEll, that I cannot give. I read your vent.

I think its a big pile of hoooey. No you do not need to take on his kiddo unless YOU choose to, and its ok to say no to that.

He lied. Read that again, he lied. Knowing full well you would walk away and not give him a chance, he lied to you about a child that he basically abandoned.

I would say that you and he have bigger issues than if you are going to spend weekends with a kiddo thats not yours.