If it were me, I would just be honest.

"It's important to me that she's able to see her family members and I'm willing to help make it happen. That said, finances are tight and it isn't feasible for me to pay for her travel costs." It is a little awkward because it should be your x covering the travel expenses for his child to come spend the holidays with his side of the family (not your x-sil), but she's the one who's reached out to you.
Maybe offer what you are able/willing to do - "I can get her to the airport/drive her to x spot/cover xx% of her ticket, but I won't be able to cover the cost and make the arrangements by myself. I'll need to speak with x directly to figure out what he is willing to contribute to this trip."
If they've asked for her to come out, it may be assumed that they're going to cover it for her anyway.

I don't know what their personalities are like, though. Good luck!