Like I said, don't tell the boys anything bad about their mother, all they need to know is that it didn't work out and you are divorcing, and that you are trying to remain civil.
As for your family, you would not be taking that from her. Her actions are her actions and whatever consequences flow from that, flow from that. She made choices, and choices have consequences. I am not saying to go scorched earth on bookface or anything, but since your mother will primarily be the one you don't want to disappoint with your actions, let her know that your actions did not precipitate this and why. Your mother can do what she wants with that information.
Your parents are on your side, and should remain so. If they want to forgive, which you can advocate for if you like, then that is their choice. If you just want to explain to your mother, you can always ask that it be between just you two and not be spread around. Don't be more concerned with Martha's feelings than your own feelings or your mother's.
Edit: I personally wouldn't want the ex showing up at all holidays and family events, maybe an occasional drop in would be ok. At some point, you are gonna bring some chick or other to Christmas or Thanksgiving or the satanic holidays like Halloween and Easter, so it might be a bit awkward if she is there (or brings some butthead or other with her). Not being married means you don't do married things together on purpose. Your ex can go see your family when you aren't there is the way I see it. She can go travel and see hers if they aren't local.