Like you said, if she is gonna be that way with your cousin when she has a boyfriend, there's really no reason to trust her to be any different with you.  Many people tell themselves that it is only because her boyfriend probably doesn't treat her the way she should be treated, or he ignores her, but really she may just be a big flirt who is constantly getting close to boys.  Not my place to judge, but she sounds undeserving of your love or anything more than being an acquaintance.  Just seeing her do that would shake my trust in her, and would make having her as even a friend sort of hard.