My ex slept with his brothers ex

Just need to vent
My ex and I had been broken up for 4 months when I found this out.

For context he worked with this girl and she was always an issue in our relationship. She constantly lies, cheats in every relationship, gets with people’s boyfriends just an all round mean girl, I’ve never met anyone like her.

Anyways while my ex and I were still together his brother had been sleeping with this same girl for about 6 months. I’m not sure how it ended but her being the girl she is, he attempted to take his own life because of her. I still remember the night everyone was out looking for him, and even filed a missing persons. Luckily, he ended up being okay.

Fast forward and my ex and I broke up a month after this situation with his brother and the girl. (We were together for 2 years)
Couple months later I find out my ex has been at her house late at night, nearly every night. So yea they are sleeping together.

I just feel so disgusted, I didn’t know him at all, I had no idea he was capable of doing something so morally wrong. I don’t even know if his brother is aware. It’s been about 6 months now and I’m over him, I want nothing to do with him but this is still in my mind a lot. I don’t even care that he slept with the girl who was always an issue, it’s because he still slept with her despite everything with his brother.

I just needed to vent but if anyone has advice please let me know, I feel like this is the one thing that still keeps him on my mind and I hate it.