No adult has the right to lay a hand on you, no matter how pissed he is. That's not just a matter of morality, it's the LAW!
That said, I can see that all this is an extremely high tension situation, and everyone is trying to learn how to relate and cope with it. There have to be clear boundaries, yes, but it must be very confusing and stressful for both Sarah and Brad.
But again, please don't blame yourself. You are the one who needs support and understanding.
Nobody living can ever stop me
As I go walking my freedom highway.
Nobody living can make me turn back:
This land was made for you and me. (Woody Guthrie)