Dating someone your cousin had a fling with?

Ok so.... sad to say my cousin is very promiscuous. She has slept with or had a fling with so many ppl. So this guy that really likes me, who i really like back, They had a fling I don't know if they just made out or had but personally i don't want to know. especially since i've started seeing him. no we have not had and we have been seeing each other for almost a month now. she flings herself on him when he's drunk but he pushes her a away and shows me any messages she sends what ever the content. He said he wishes he met me first and hopes that the fact that my cousin was attracted to him and that he want initially attracted to her doesn't ruin his chances. My friends all agree that he is definately into me and i really don't know if i should feel guilty. seeing that she wants to sleep with the world and claims rights to them. i dunno. i'm confused.