Mother in Law issue - Opinions?

We are having my daughter's first birthday party and my mother in law has chosen to go to a friend's daughter's wedding instead. That is not the worst part - We had already discussed/resolved the wedding/party conflict this with her 3 months ago and she said she coming, lying all along, knowing fulll well that she had already booked her tickets in advance. Now she drops the bomb on us a week before the party. She says she never said yes, but rather said "ok" . She said it only matters that she is around on her real birthday, which she won't be because my husband and I are taking out our daughter for a special day as a family. This is not the first time my MIL has dropped us for a better offer. It appears as if the novelty of her granddaughter has worn off. Don't even get me started on what she did before our wedding...Any opinions?