Is it ok for your partner to invite his ex-mother-in-law to come on...

[bellabella]

...vacation with you and his kids? I was told after the invitation had been given. She's decided to come. He says it's ok because she's the kids' grandmother. I'm upset about this, as she's his ex-wife's mother, and I wasn't consulted. I am happy for her to be around lots of other times - just draw the line at coming on vacation. Am I being unreasonable?