There seems to be some magic with my mother in law riding in the back of my Model X. She is more calm and mellow. I teather her back there and she seems to like how low the floor is. Kind of like a cave for her to sleep in. I put a towel over the rear bumper and she easily crawls right in and out. For some reason this is a happy place for her.
You make your Mother in law sit in the cargo area? Do you give her rawhide too?