I'll preface this by saying that my brother and I are very close and he dated an AMAZING girl (who treated him very well). That girl and I became close friends over the years and I came to think of her like a little sister (actually I am closer to her than my own sister). After they broke up, she and I stayed friends and, years later now, I consider her one of my best friends. I do compare his new girlfriends to her, but silently, as it's not my fucking place to say a word about her to him.

Your boyfriend needs to have a talk with his family. He doesn't even need to reveal that the ex cheated if he really doesn't want to. Something like, "I'm glad you supported me so much when I dated Ex. However, we broke up for very real reasons and, while I'm not comfortable sharing those with you, I will only say that she did some extremely hurtful things. I would hope and think that your main concern is my happiness. OP makes me happy. She treats me well. I trust her. She makes my life better. Since I am so happy to have her in my life, I would really love it if you could try to support our relationship. Unfortunately, if you can't, I see us spending less time together, as I want OP to feel comfortable, and that's not something anyone wants."

Their main priority should be his happiness. Otherwise, they're just being selfish. I also didn't see you mention yourself putting much effort in, so maybe you could try to be friends with them? They probably sense that you don't like them as much as the ex did. You say you don't "click" but maybe you could try again? If he's someone you see yourself with for a long time, they are going to be in your life.

Anyway, if they can't learn to respect you and STOP mentioning the ex, it may be time to distance yourself from them. You do NOT need to hear about her from them.