What The BLEEP am i to do ?

So i've been dating this chick, who is like primo sexy hottie. but people say shes a party girl. And I went to a party with her, and her sister. Well i was hammered after awhile, and Im pretty sure I saw her making out with her sister while guys watched. So I literally don't know what to do. should i confront her about it, or break it off, or stay with her. I can't even begin to know what to do someone plz help