Nothing bro, I am afraid to say that in my experience watching other folk try to navigate similar situations. Stuff will not change and if that gal you like tells the daughter to stay home and watch her own kids she will resent you for ruining her golden deal. That is when the probs come in.

My buddy was in a similar situation with his girlfriends step sons (Their dad died and she raised them from teens) who every friday thought she was there to watch their kids. Well in comes my buddy who wants to go out on friday night with her. She told them she was going on a date on friday night and ever since then they have tried to torpedo those two's relationship. They were going to get married but have since said phuck that as it will never work. now they see each other occasionally even though they really love each other. The drama and resentment from all parties involved is not worth it.