My boyfriend and Sister hate each other?

So to sum up the story is basically my sister has hated my bf from day one. Understandably she raised raised me for 2 years (due to other family drama), and she doesn't like to see me get hurt. Now he hates her because she used me to babysit and disapointed me a lot of times. Now this all came to a head where she basically disowned me, and threatened to have him beat up. He started **** on facebook (immature for a sister 10 years than me). I was obviously distrought about it. Me and her didn't talk for 2 months, and now we are trying to make amends, and I miss my neice and nephew more than anything. He feels stabbed in the back, understandably, and is incredibly upset with me and wants to break up with me. I'm stuck again between this tug of war between them, and have no idea what to do. Any advice would be greatfully appreciated.