How do you get someone to stop bringing up an ex?

I had a situationship with this guy two years ago. He was pretty terrible and treated me horribly. He never wanted a relationship but also didn’t want me dating anyone else. It only felt like he was just chatting to me when he was not getting attention from another girl. I didn’t know this at the time.

He was friends with my sister before we met and they had some good times. They were coworkers and they would go eat somewhere together for lunch and talk. Him and I met through her and it was his idea supposedly. There was always drama between the three of us.
He got into a relationship with another girl and that’s when he started treating me even more terrible including my sister but then later would come back like nothing happened. I ended things with him for good to respect his relationship though he didn’t want to.

Now when I’m hanging out with my sister, she brings him up randomly. One minute she’s calling him gay or DL the next minute it’s a funny story between them two with a smile on her face. It’s been two years and I don’t feel like being stuck on him. There’s nothing “😊” worth about him and I regret everyday being involved with him.

How do you get someone to stop bringing up an ex?
Post Opinion