Your home base in every Pokémon game always has two stories: your penthouse suite upstairs (complete with the latest Nintendo system) and the living room/kitchen downstairs. So where does your mom sleep? In the kitchen?!
Get used to Game Freak logic. They didn't build a room for her so she may sleep in your bedroom while your gone. But like I said Game Freak logic :3
Your mom is probably not the richest person in the games since she has to take care of you all be her self. She hasn't seen her husband for a while now and hopes he comes back soon. It is possible that your mom sleeps in the same bed as you or just on a matrass on the floor. This happens in real life too.
I know this question has already been answered but it may be as simple as there are other rooms upstairs. You only have need to be in your room so the games automatically lead you there. This may not be the answer you are looking for but it makes sense in a way.