What can i do for him?

I can't believe i'm resorting to asking complete strangers for advice but maybe since your not familiar with the people you can help. I really like my best friend. A lot. And he has started dating my younger cousin. Now i love my little cousin, but i know how she is. Number one, she's like a pathological liar. She lies about things totally unnecessarily. She's also been known to cheat on her boyfriends and do some really WILD things with other guys. I'd never really worried about it before, but then again, she's never dated a friend of mine much less my best friend. And i think i may have feelings for my friend. I'm not sure if he returns them, but i can't tell him anyway. I feel like it's not my place to tell him about my cousin and she is family, but i love him very much and don't want him hurt. I'm also afraid he won't believe me or maybe think i'm jealous. Actually, i am a little jealous, but i just really don't want her to hurt him and i'm pretty sure she will. She's sort of promiscuous. Ok really promiscuous. I'm just confused. Should i tell him about her or should i just let him find out? And should i tell him how i feel about him, even though he's dating my cousin?