It's surely big enough that she could have converted the room to a bedroom and put a wall down Rex's room to create a hallway so that his en-suite became the family bathroom instead, giving a total of 5 bedrooms upstairs - then the boys wouldn't have to be in the annexe!That bathroom is a decent size since they removed the cupboard. She could have made it an actual bathroom instead of a shower room.
That house could have been made to fit her family properly but her skanky wanky shell sinks are more important I guess.