mother of the bride, not included in rehearsal dinner

Sounds like a family feud erupting even before the couple gets married! Exclusion of the mother of the bride (assuming she is physically able to participate) is absolutely unheard of. If the groom and his parents truly hope for a their son to have a happy marriage, the last thing they should be doing is insulting the bride's mother. Some really weird dynamics going on here.

Ask your daughter and her fiance to intervene for you. It is their wedding and he should really be the one to talk to his parents about this. If that fails or they are afraid to speak up, I would contact his mother directly and (calmly) explain how much this means to you and perhaps offer to pay for your own dinner or help with the funding of the rehearsal dinner if you can (in case money is the issue, but I doubt it is.) Most importantly, if you would like to have a good relationship with your daughter, please dont boycott her wedding. Have the couple over for a meal at your house in preparation for the big day and enjoy the wedding (at a safe distance from his monstrous parents, of course!) I'm sure it hurts, but you dont want to hurt your daughter's feelings or hurt your relationship with her.