I had the same issue with my boyfriend's sister. But, after giving birth I completely changed into a whole new person. Everything that happened in the past was the past & I guess becoming a mother made me a better person for my son. I talked to my boyfriend's sister like hi & bye or how you doing? & blah. But, till this day I still don't trust her or even have respect because she called me B***tch & a dead beat mom. I hope things get better for you!(:
Be the bigger person for your boyfriend. Keep your friends close and your enemies closer. Get along with her but let him know that you're setting some tight boundaries and if she crosses it, that's it. I know what you're feeling, I've been through it too. But, but be the bigger person for his sake. Relationships like that are important to a man.