No need to get sarcastic with the OP, she has done nothing wrong and seems very family minded and like she’s trying to do what’s best for the little girl and her fiance.
Its completely absurd to think that in getting married he’s abandoning his responsibilities. He can be a father and a husband. And giving his child new siblings is a good and joyful thing, not a punishment. If the OP is determined to be a good, attentive, and supportive stepmother this child’s family will grow, and there will be more people to love and be there for her. If her mom really is nuts the child will have someone to turn to for help.
I was abused by my stepfather, but even I can acknowledge that many people love their stepparents, and that they are often a blessing to the family and children. Of course they need family counseling to make this as smooth as possible, lots of communication, lots of patience, and plenty of understanding. But if they are determined this can be a great thing for everyone. Actually, I think it will be easier because the child doesn’t live with her father full time. The stepmom can kind of be a gradual transition.
He should not have to put getting married and having more children on hold for 18 years because he already has a child that he only has joint custody and/or visitation with. Especially since any issues that arise can be worked through.