I think it was hurtful to the FIL’s daughters and also very petty of MIL. Why not have all of you go together? Were the daughters guests in her home at the time?
It doesn’t make sense to me to not invite them. She sounds a selfish woman who I would avoid. All you can do is behave politely, shame her with your civility, and avoid her whenever you can. I would also extend a friendly hand to these daughters, just so they know that it’s not a them vs us thing. But maybe they already know that.
Trust me–I have a MIL just like her (except she doesn’t gamble).