She started peeing indoor while living at my mom quite recently from what I can understand. And there she has the freedom to go outside if she wanted.
But maybe the colder weather makes her know that she will be taken inside. Although, when they have tried letting her out in the winter earlier years she just shivers and runs back in. I agree that moving the cat between 2 households is bad, but I guess it looked like she felt at home in both places.
Maybe she doesn't understand Swedish climate? It is very warm in the summer and super cold in the winter. It is not really T-shirt weather anymore, and the cold should come soon as we get deeper into october.

But yeah, told my brother to call the vet. I hope he does.