I can certainly understand how you're feeling. The ex and her family are acting AWFUL. I don't know who instigated THAT divorce (was it him?) but regardless, it's OVER and she needs to get over it.

He has remarried, and she needs to move on. The family shouldn't even be putting their noses into this. It's none of their business!
As for the child, it sounds like she's quite mature for her age, and she's getting pleasure out of it.

By acting snooty, I'll bet she KNOWS it burns you up, and she does it on purpose. But STILL, she's just a child and children don't know the whole picture. They don't have a clue as to what is going on. And children are ALWAYS the ones who get hurt in a divorce. This child is being used as a pawn. Mom is telling her to say things to Dad, and Dad is telling her to tell Mom things, etc. Then when she comes to your house, she acts like a little snot -- I imagine Mom told her to, when normally she's a good kid!
I feel sorry for the child. I doubt she knows WHO she's supposed to align with.

She doesn't know who's "right" and who's "wrong" or what anything is about.. She's totally confused and doesn't really know what divorce IS.
But why doesn't your HUSBAND'S family recognize the marriage just because the ex is causing problems? Confusing.
I have to agree that seeing a therapist would be the best idea. Your doctor can refer you to a GOOD therapist as he would know the best one for you. Please take to your doctor about this, and have him refer you. I know you won't regret it. Whether or not you leave or stay with your husband, you still need therapy in order to rid yourself of the resentments. There is a saying: "Resentment is the poison I take to kill you." And it's true -- it eats you from the inside out. It's like a cancer, and it doesn't stop unless you seek therapy.
I wish you the very best. Please keep us informed as to your progress, will you? God bless and please take care. Hugs, Lee