My boyfriends mate

My sister was seeing a guy who is best mates with my partner of a year and a half, and long story short, he was leading her on and seeing another (older 🤨) girl for 12 months, seeing my sister for approximately 9 months.

I asked my partner if he knew about it and had met her, he said no he didn’t know and he seemed shocked. He has another mate with the same name as this mate, who’s partner has the same name as my sister, who is actually friends with my sister and I get along with her well too, so my partner wasn’t sure which couple I was talking about.

Seemed genuine but he is a really ride or die kind of friend and this dude who was seeing my sister is one of his best mates, so I do worry he isn’t being completely honest with me. But then I have been cheated on and I have a hard time with trust.

I told him I’d be pissed off with him if he knew, but at the same time I mentioned how everyone already knows that he’s always been caught up in his ex and so my sister was doomed from the start. Which my partner agreed with and said she talks about him all the time. My partner did tell me very early on that he doesn’t think my sister should bother because he’s still hung up on his ex.

What do you do in this situation?