Some Personal Advice Needed

Family FIRST. Period.

Sounds like your friend and his fiance aren't on the same page here...

If she can't accept that then mabe she wasn't "the one" to begin with.

This is IMHO, of course.

Blood is thicker than water. if the fiance doesnt like it, then tell her to start packing.

Indeed blood is thicker than water - Fiance has to go

Pure and simple

Thanks for the replies everyone, but here's the thing...

He has 2 kids with his fiance. He words are "should I break their hearts and tell their mother we're done to make me happy?" It's really a tough one.

Aren't multiple IDs banned?

Why are people creating new id's just to ask a question they must be embarrassed to ask with their other ID? If you're too embarrassed to ask under your other username(s) maybe this is the wrong place for advice.

As far as I know multiple usernames are banned, especially since they have been used in the past to provide fake testimonials for someone's own wso product.

As for your friend, it's his decision, not yours or anyone else's.

Well, if his fiance really love her, he should accept the niece for whatever reason is.

+1
Yes, really.

This is a very serious "life" test for the relationship. I don't know what the fiance's family is like, but if she has any siblings with children and something "happened" leaving her with the chance to "rescue" her own from a traumatic situation - would she?
There are some very deep moral, family values and mind set issues that may or may not have been examined by these two before.
Now, your friend has already established a bond with this member of his family. He's raised her, cared for her and wants to protect her. He obviously has a very strong sense of family.
As for the fiance, it would be ideal to ask if she would want to go to a councilor together with the kids and your friend. Try to see how the kids feel about it. Many young kids like having older siblings. The fiance may have issues relating to siblings or even family problems in general. Her relationship with her own family might be a clue to this problem of acceptance.
What your friend needs to explain to her are his feelings for her and the children. Then also stating his feelings for his niece who is like a daughter. She may feel insecure about her importance to him. She may be jealous of the closeness or commitment that he shows towards his niece. In the end, his heart is going to whisper what is right for him. His fiance could try to find out why she's so against this - that it might be caused from something internal - from her past. Her rejection having nothing to do with the teenager.
What's going to happen when their children become teenagers? Is she going to run away from that issue too? Present her with an idea, that discovering what she can about parenting a teenager could help prepare her for when their kids grow up.
She should accept this chance to have someone around that could even offer her some companionship, assistance and even friendship.
I've several children of my own, have taken in even more as the kids grew up. It's surprising how responsive young children are to those older. They learn to do things without a fight cause they want to be "like a big kid".
One of my fondest memories as a young child living with my aunt for a time, was of my older cousin. He was taking care of me while my aunt was busy in the kitchen. He not only invited me into his room - a rare treat. But he had me join him on his bed and read to me from the many comic books that he collected. I don't recall much of the stories he read - but he read them with a flair for the dramatic.
He eventually went into radio when he grew up. But I treasure that memory of him.
Perhaps this fiance needs to really search her own heart and decide if she is willing to commit to the reality of marriage. Because situations happen in families - you can't just pick and choose what you want to deal with. It's an all or nothing kind of contract. If she tries this my way or the highway ultimatum - she may lose everything she's worked so hard for. She has a guy that is dedicated to family.
What woman wouldn't want a guy like that! If she tries playing the us or her - she may not like his answer.
He shouldn't try that either - these children are his too. He also needs to make sure that his lady doesn't make the niece feel like an intruder. She's either a part of the family or the negative fallout is going to hurt her too.
The idea is to offer her a home - a real one, with a real family that wants her.
Compromise is the only way - if you're going to commit to a life together, that's a big part of the deal. No one gets their way all the time. And with something this important there has to be some give and take.

Look at it this way - with a bit of humor...if he has to put up with her mother, then she should give a little and accept his niece. Once they get to know one another they may find that there are less issues than they imagined.
Love, tolerance and caring...they can overcome a lot of things. Give it a try.

Wish your friend and his whole family the brightest of blessings. Most of all - patience, understanding and lots of love to them.

There is no way on God's green Earth any woman I'm seeing is telling me my children CAN'T come and live with me.

Eight years and two kids and they 'plan to get married'...........
I smell a bit of a phobia to commitment somewhere in there....dont know whose
He had the niece when they met and it was ok then but not now?
Personally, having a teen might be a big help with the little ones I think.
'Assuming' a teen will be 'trouble' could become a self-fufilling prophecy, and the fiance needs to figure out what the true 'issues' are with it all.

Something deeper is going on, and seriously, some counseling that includes them all is probably the best way to figure out what it really is.
If the teen is 'troubled'...all the more reason to be there for her and help her become a better adjusted adult.

good luck
Andie