As devil advocate, it could be that OP is worried that even if they say WHY they broke up, people will just accuse them of slandering the ex.

Pretty sure that is the perfect reason to tell them. Maybe they'll shut up about her.

I would tell them. After all, why protect her image when she didn't even care about you?

I don't understand you, OP... Why protect a cheater. I'd scream to the world that X is a cheater if one day a guy has the audacity to cheat on me. Hell hath no fury and all that jazz... and seriously... your excuse to not telling your family is rather pathetic. Have you no self worth. These aren't even mutual friends between you and the ex. They are your own flesh & blood. They should believe you.. however, letting them indirectly berate you so much before saying something might cause suspicious. you didn't **** up dumping your exes' cheating ass, but you totally ****** up by taking a fall for her. Seriously can't think why anyone would do this. Are you fond of your pre-cheating ex to the point you want the "happy" memory to live on or some weird shit like that? just... just... why?

Tell them to get over it and stop being insensitive. She wasn't in a relationship with them, your relationships aren't any of their business and family is supposed to support your decisions.

Having been the cheating ex girlfriend a long time ago, it probably means a lot to her. You're a good guy. You probably loved her very much, and that's nice to see.

tell them and you'll never hear about her again.

Umm...tell them?

tell them so they will not blame u anymore

Tell them, screwer