It's pretty selfish of him to want to take SD away from her mother, so he can "raise her to his standards." It shows a total lack of empathy for BM and for SD, quite frankly.  Does he really think he's so awesomely fantastic that he can cut her mother, a "fine parent" in your words, out of her life and there will be no consequences?  And does he truly believe that he can just replace her biological mother with you? How would he feel if BM proposed that very same thing in regard to him? That he just go away so she can replace him with her current boyfriend.

It's incredibly narcissistic and verging on parental alienation. AND, it's impossible. No court is going to hand him full custody if BM is a capable parent.

Yuck.

You said yourself that BM is a fine parent.  Why in the world would your fiance want to take his daughter away from her mother if BM is a decent parent?  That makes no sense, and unless for some reason BM wanted to be an EOWE parent and agreed to it, it's not going to happen.  I don't think there's anything wrong with you wanting to keep 50/50 custody so long as BM is willing and able to have half the parenting time.  In fact, I think it is the right thing to do.   

My SO has 50/50 custody of his daughters and I am A-OK with that.  He would love to have them all the time, but he knows that even if BM would go along with that (she wouldn't), it wouldn't be fair to the girls because they love their mother and want to spend time with her.  They love me and enjoy spending time with SO and me, but we would be delusional to think that I could (or should) replace their mom.  SO also realizes that having a break from the kids is good for our relationship, and he's pretty good at prioritizing the relationship.

The time for him to choose the mother of his child was BEFORE he had a child with someone. He doesn't get to choose after the fact, and frankly, it's worrisome to a large degree that this is his philosophy.

I want you to take SD out of the equation for a moment and look at your own relationship, and your future. You're not incorrect in hoe you view any of this, so there is no need to focus on that. You're right, he's wrong, and it doesn't get more cut-and-dry than what you have described.

What I want YOU to think about, though, is whether YOU feel like you are going to be THE wife or the REPLACEMENT wife. We talk a lot on this board about how we "second wives" should be treated as THE wife, because that is what we are. We are our own person with our own goals, quirks, upsides, downsides, etc. We aren't an upgraded version of BM. We may be an upgrade from the previous relationship, but we aren't "XW/BM Version 2.0".

It doesn't sound like your FH sees you as THE wife. It sounds like he sees you as the replacement. He sees his first marriage as a failure, and instead of cultivating a new relationship and growing from his mistakes that fed into his divorce, he is just going to replace the piece that he thought was broken - the wife/mother.

That is an unhealthy thought process on a multitude of levels. But let's look and see what that means for your future. It means that 1) when you no longer fulfil his fantasy, he'll replace you, and 2) if you have any children, he'll try to replace you with the next wife.

These are some serious issues that you all need to address - and you need to seriously consider what ACTIONS he would need to take to prove that he won't throw you away when he doesn't get what he wants. He needs to work through his own guilt at the failure of his first family before he even considers dating, much less getting married. He's not just raising red flags; he's a bright red, flashing beacon screaming "DANGER WILL ROBINSON, DANGER!"

My advice? Postpone the engagement indefinitely. Don't push the wedding back 6 months; make it clear what he needs to do to prove to you that he 1) has dealt with his guilt and 2) will treat you as a partner and not an accessory.

He has unrealistic expectations, and I implore you to keep saying no. Say no to helping him get sole custody of his daughter (he won't, by the way, and he'll dump any mutual money you have into an attorney for a fruitless effort). Say no to taking on more of a parental role. Say no to loving her like your own. Just keep saying no if it doesn't feel right. Your instincts are spot on, and it sounds like you know yourself well. Don't stop listening to yourself just because it will keep him around.

He should be in love with you, want to be with you, want to do thing with you.  You should not be the new 2.0 new mother for his child.  Red flag.  Child has a mother, doesn’t need two. Nothing wrong to have fun with the child, and guide her in your home.  But you can not mother her, because she has a mother. You are with a person with a mental problem. 

You need to be looking for a better option for your equity life parnership. 

You get it. He doesn't.

Good luck.

his measure of success for the relationship is if we’re just like his first family

Don't you think that is kind of creepy? He wants YOU to fit in to what he thinks HIS former family was? Why in the world would you want to have anything to do with that? He sees you as a puzzle piece, not your own individual person.

I hope couples counseling helps him with his totally unrealistic and unfair expectations. But, man, I'd be totally stubborn about keeping my own identity, feelings, and desires in tact.

As a SKid from the '80s and a SDad who has a couple young adult offspring who still live at my Ex's house (and these kids are 20 and 23, still no drivers licenses because their mom "refused to allow" me to "rob (her) children of their childhood"), I have come to a lot of conclusions. 

My ex wedged between me and my kids, and I look back and with I would have forced the schedule.

DW's ex seems HAPPY to give away his kids, seems to WANT that. While we take them in, there are times I really think this bastard needs to have his kids around so he HAS TO have a relationship. But there's something about leading horses to water, so..... If he wants us to take his kids MORE, then I'd say, "COOL. Say it. I'll happily agree, and I'll happily be an integral part of their lives, however you'll need to cough up some additional child support." Which I know this bastard won't do because he's a cheap sumbitch - which brings us back to square one... So I'd love to tell him, "Take the time and enjoy your time with your kids, fool. Believe me, my daughter is so manipulated by her mom that I consider her lost in the woods. You could lose them to a crazy person, and then you will really hate that you didn't do more."

To this end, I say to all who would hear me, as a parent... as a step... don't be selfish. this is about the kids developing healthy relationships, too, and your SD needs that with her mom. She needs to be able to do 100% of her half. You need to be able to do your 100% of your step half. Doing more would come at the expense of others.

Just as an aside... I think your husband's wishes for you to become essentially the replacement mom is unhealthy. Even if the biomom was in prison, you should not be seen as a replacement. The last thing I'd want is an expectation to be superdad to a couple of kids I hardly know, and for them to be superkids to me. Woahhhh, hold those horses... I'm awesome, but those kids need to know their dad as DAD, and me as the guy who keeps mommy's wine glass full and stuff like that... Anything I do is BECAUSE I LOVE AND CARE, not because it's my court-ordered responsibility. I choose this, and I think it says a lot about Steps who do it anyway (shout out and a fist bump to all my fellow steps who see the open door but choose to stay and do the important stuff as long as there's a healthy chance to make it work).