It's natural that the mother lion in you roars when someone hurts one of your children, even if the offender is another one of your kids. But unless you address the underlying cause of your son's torments, things aren't likely to improve.
The idea that your parents might have liked or like a sibling more than you is tough to take, sure. But know that parents can love all their children, even if they favor one over the other.
I was shocked at how quickly the boys blamed each other. And I know this was a tiny, almost meaningless event. They weren't screaming, breaking each other's stuff or storming off in a huff. It was a far cry from fratricide. But their united front was cracked. That had never happened before.