Help! my cousin almost raped me.?

Okay so I was at my sisters & they were having a party for her husbands birthday. There was drinking of course but It was all pretty much family & friends. I'm 18 and yess I was also drinking (yeah I know underage drinking is bad no need to tell me) I knew everyone there so its not like I was drinking around people I didn't know. Well after alil I started feeling sick & decided to go lay down. ( the party was in the garage out back) I wasn't trashed but I was deff feeling the effect of the alcohol. I don't know if he heard me say I was going into the house or if he just happened to go in but this is what happened: I was laying on my sisters bedroom floor (for some reason it made my stomach feel alil better) well he came in not long after I had went in and sat down beside me. He asked me what I was doing on the floor and then tried picking me up & told me I'd feel better if I were on the bed instead of the hard floor. I told him I was fine and I just wanted to lay still but he insisted and wouldn't stop bothering me so he kinda helped me on to the bed but then he like stayed crouched on top of me for a moment & I asked "what are you doing?" He told me I was so pretty & was such a sweet girl..I remember telling him thanks & already feeling creeped out so I tried to squirm away from him but he garbed my arms and pined my legs with his and started kissing me like all over my face and neck. I told him to stop but he didn't until he heard the front door open. (it was my sister coming to see if i was okay) He jumped off and ran out of the room as if nothing ever happened. I didn't tell anyone but now being around him makes me sick. I still wonder what would of happened if my sister did not come in. I know that he was drunk but its not the first time he has acted weird. Never did he do something that extreme though. I know I should tell but I just feel so guilty about it...my cousin really loves him. she is finally happy & i don't want to take that away form her..should I just ignore it since he was drunk after all? or should I tell? for those who are wondering he is around 30 &He is my cousin through marrige not blood