I do not recommend they share a room with her daughter. You never mentioned there is a child involved. You did mention a cat. We can only go by what you post.

I see how they might need two bedrooms. Still not 3. They might want 3. But they don’t need it. Unless they have some other reason that you didn’t mention. Like her aunt and uncle living with them and 2 cousins. Or she is currently pregnant with triplets.