It’s the animal’s home. I am incredibly far away from being one of those “pet parents” with “fur babies,” but other people have absolutely no right to decide where you keep your pets or if you have them. My mother is a big “no pets in the house” person. Always has been. When I had to move back temporarily I was allowed to keep my bird and rabbit inside in their cage and pen (which greatly surprised me honestly), but my dog had to live outside, and I wasn’t allowed to bring my bearded dragon. Her house, her rules. Since I moved out, I’ve got a cat, and I have fostered some dogs, all which were allowed inside. And when my parents visit, that doesn’t change. They don’t expect it, or even ask, because they aren’t ridiculous and entitled. They know this is an indoor pet home. OP’s MIL knew their home was an indoor pet home, and still wanted to move there. You don’t get to make up the rules for the homes of other people.