Kind of long im sorry advice please

I've told my fiance time and time again to not be around my cousin BC she's fake and talks about me. She can't be trusted she's slept with her sisters boyfriend's, her best friends baby daddy it's just a mess . today what I predicted happened. He butt dialed me and I heard her talking about me to him and instead of him defending me he was just laughing it off. Long story short we argued about this the other day I told him it would come down to this which is why I didn't want him around her. Her boyfriend & my fiance are close but that doesn't give him the right to chill at her house especially if I'm not there. I'm not sure if it's my hormones and I'm overreacting or what. Would you be upset in this situation