Okay, this happened last May... that's a while ago. Also, you can't control what your inlaws say or do or when they do it. If they bad mouth you to your parents, eventually it may be to your neighbors, or your husband, or when your kids are older, it will be your kids. That is behavior that is going to be firmly nipped in the bud by you and your husband talking to them about it. Really, I think that banning any future visits that include them and your parents is juvenile and overreacting. Your parents have the option to walk away and say, "I'm not going to listen to this." Your parents were under no obligation to promise to them they would keep anything a secret. So, hat is an issue in it's self.
And yes, I can believe they bad mouthed you to your parents. That kind of stuff happens all the time. I have no clue if they do it for spite and plain meanness and just make stuff up, or if they really feel justified in what they say. I have a hard time believing that a relationship is totally sour just because of one side, though I know it can happen. I had a toxic relationship with my inlaws for years, when I came to the realization that I was being too nitpicky and too easily offended among other things. When I decided to work on our relationship and my perspective, everything changed and now we are very close, though some things they do still annoy me.
I would suggest you read the book "How to Hug a Porcupine: Dealing With Toxic & Difficult to Love Personalities" by John Lund. Some excellent advice there. Because some people really ARE crazy and mean without any sort of provocation.
*Just wanted to add... I obviously have no idea what they said or what they have done in the past, so I don't know severity wise the details. If I felt having them around at the same time as my parents would create a possibly dangerous time or they made large scenes that make the children visibly upset, then I would not have them there. If it is something that is contained quietly on the side and can be handled gracefully by the other adults, I would still invite them, though they would have a warning prior.