They used to have a class together in highschool and they weren't friends but they got along fine. I think it's my sister's insecurites making her irrational. The first guy she dated didn't have a gf before her. This one did. I think she's afraid that he will leave her and go back to
Claire.
It may be partly exacerbated by that fact that my ex-bf cheated on me with his ex-gf and ended up marrying her, but she needs to realize that not all guys are like that.