Advice please!!!!

Okay so my boyfriend doesn't have a very happy family life and he recently found out that he is family of one of my friends... They dont know if they're blood related and they havent even known each other that long.... 
Also my friend is a little bit on the slutty side and even flirts with her own cousin to the point where people always ask if they're dating... So is it wrong of me to not be happy with the whole idea of them becoming closer?  Or is it just the PMS talking.... 🙈