grannypiper
That is awful. I wouldn't make a fuss but the GD would never be welcome in my home again.
If that's you not making a fuss, I wouldn't like to see you with your hackles up
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That's the sort of attitude that splits families, and is likely to leave you hosting events with no guests, as if the granddaughter is banned, when her own children come along they won't be visiting either, and at family occasions it is probable that her parents will boycott too, out of solidarity.
OP, I can understand your feeling hurt, but if spaces are limited it could be that the couple are in an impossible position. Also, as others have said, would your partner's ex be there, and if so, would that be an awkward situation?
Can you make the most of having the house to yourself, or arrange to go somewhere nice with a friend for the day? You will be saving money on an outfit and hat, so you could smash out on a spa day or whatever is your idea of a good day out.