My boyfriend is cheating on me with my sister

I am so upset. She is 2 years older than me and I have always suspected that he liked her as whenever he would come round he would get all nervous around her and ask me whether she was hanging around for a while, also he was always trying to subtly ask questions about her likes and dislikes. I teased him and he started looking uncomfortable but laughing it off.

Anyway we just all went to the club. I didn't even know my sister was going to be there as we don't really get on that well. My boyfriend was already completely hammered after the house party we were at. I was really angry with him and said he should stop drinking and embarassing me. I told him I wasn't going to spend the night with him and got the taxi home. He just texted saying "dont care if you're gone I am having more fun with your sister hahaha" and I texted replying "whatever" anyway my friend just called and said I really don't know how to tell you but I can see him hugging your sister and she is on his lap but she said she can't tell if they are kissing. I just told her to go up right now and check I am waiting for her reply. I am fuming. How could they do this to me. What the **** do I do???? Should I go and get him? I have asked my friend to take pics but I don't know if she can

First of all dump him and secondly you gotta **** your sister of as well because that ain’t family. You’re too good to be treated like that, you’ll find someone who’ll do anything to be your everything