what do i do?

okay so, I've been dating my boyfriend who recently proposed for three years. he is my "cousin's" best friend. I say "cousin" because we're not blood. my uncle dated his mom for 6years off and on but they are no longer together. I've gotten really close to my cousin over the years. And we hang out all the time my boyfriend and I practically live with him. but my boyfriend is always freaking out on me saying that my cousin doesn't see me as his cousin and constantly says that my cousin wants to fuck me. I know that he's scared of that given thier history. but no matter what I do or say, or my cousin does or says he still freaks out about it. yes there have been a few drunk nights when I've thought about it. but I love my boyfriend and I do see my cousin as my cousin so I would never act in those thoughts, even while drunk. But I can't help but think about it.