I REALLY don't know what to do?

It's not your fault. If you want to continue going out with him, keep going out with him. The only way you're going to get caught up in drama is if you want to. Just stay away from everything and act like nothing's going on. Date your guy, act normal with his sister and her friend, and go about your life.

By the way, your profile picture is AWESOME!!! I'm going to see MCR for the third time in April!

Okay where do i even start? well hmm let's see maybe they should try the forgive and forget thing i mean hey it's his life and yeah what he did was maybe a lil wrong but u know what they should just deal with it know what i mean? sorry i don't have a better answer but i just can't get my words out even though i think i have a better answer for this only i just can't get it out right now -n- its not really your problem but ur stuck in it and u know these ppl and stuff but u probably should be there for him if not already and yeah so i wish u luck and hope for the best!!!

You have nothing to do with it, it's his problem. You just happened to find out. Are you mad he was dating someone the first few days? If not, then you should just stay out of it and let him handle it. If his sister and all of them choose to hate you, so be it. It's not your fault however because you didn't know his history beforehand.

uhh I'm stuck but my best advice is that if his sis and his sis's bff hate maybe he should talk to them and give them both an apologie

Solution: Tell his ex and sister to go stick themselves. Case closed.