It's not your fiancee's fault that her idea of a future with you has now completely drastically changed. I can understand that dedicating your life to raising a kid who isn't yours and was sort of thrust upon you isn't an easy decision to make. This sucks because neither of you is technically wrong. That being said... I've been dating my partner who i live with for only two years and if he had to rescue his toddler sister from an abusive home, especially knowing what you went through... i just cannot imagine being mean or cold to her or putting him in a position where he had to choose between the two of us. Even if she isn't happy about the baby being around, I think it's gross and a serious turn off that she has no nuturing instincts whatsoever. Even your sister had a better living situation, or other relatives to take her, I don't know that I'd really want to procreate with this person or dedicate my life to them. Life is going to throw an exorbitant amount of monkey wrenches at you... you wanna be with someone who can bend with you. And if she can't, she's not an evil person, she just may not be for you.