Sad to say but it sounds like Sandy's daughter has inherited some of her father's sadistic tendencies. Sandy should not allow herself to be held hostage by her daughter. I am also worried that the son she is presently raising is going to suffer from her preoccupation with pleasing her cruel daughter.

While in general, the letter writer should not have to be around her ex or his mother, there are going to be some times in her life, like her daughter's wedding, when she is going to have to play "Let's Pretend."

It would not be reasonable for her to expect her daughter to have two weddings, one for each side of the family. On occasions like rites of passage, whether the letter writer likes it or not, she needs to at least be there and civil if not friendly to the others there, even if that includes her ex and his mother. And they need to do the same for her.

This girl has learned from her father to be an abuser stay away until she learns not everyone needs to "play pretend".
If she wants support from you shae has to learn how to give it back.

I just love reading the letters from some weirdo who wants advice on how to proceed with his/her/its perverted lifestyle. "Hi, Margo. I need helping figuring out how to tell Mom and Dad I'm fooling around with two people who sleep with the entire neighborhood and aren't even sure what gender they are. Any suggestions?" Lovely people we have writing in here.