You know that time when you had to share a room with your sibling and you drew an invisible line across the room, you weren't supposed to move your stuff over that line or you would hear about it from your parents. The fair thing for the parent to do in this situation is the respect the boundaries and not choose a favorite. I learned that in elementary school. If you cross that boundary, then your sibling had every right to claim back what was theirs to begin with. The sibling would most likely be ticked off and result to violence. Let's be respectful of others space as we would want them to be for us.