I’m moving back in with my non muslim family and I need to know if there is a way to make the shower at their house tahir as I know they have bathed the dog in the shower. If there isnt a way, I’m going to have a really impossible time living there.

Hold the shower head, then with the palm of your hand pour water on the shower head. The using the shower head pour water on the walls and bathtub/floor of the shower. If you do that it will become pure, even if a dog was bathed in it.