When enders game came out there was a big issue about the author of the books bring very homophobic because of his religion. I made a donation for the same price as the tickets to a pro LGBTQ+ org and still went. If your MIL gets snarky just put the donation under her name and put her email in. I would do that the next day so there is no fight and it messes up the kids time.
To be fair if I see a sign saying they support Trump, GOP, or anti-LGBT I keep walking on by. I have seen many places that say if you are a bigot, sexist, or racist you are not welcome here. Those are the places I spend my money at.