So there is this girl?

i have been friends with my boyfriends cousin for almost 5 years now on and off. When I first got with my boyfriend she stop being my friend she hated it. She hated seeing us together and told people she didn't like us being together. i started being her friend Again about 1yr ago. Big effin mistake about 1 month ago she called me and told me see made out with my dude. The first thing i thought was no way i didn't believe it i asked him and he got all mad and said gross she is like my cousin i dont know who to believe anymore i asked her how he kisses and she couldnt tell me i know how he kisses and if she did do that wih him she should know too RIGHT????