It doesnt matter if your DS lit a molotov cocktail and firebombed the SD slippers, the SD had NO right to scream and shout at your DS, had no right to frog march him anywhere and behave in the manner he did. Your SD is the adult here, and YOU are the parent.
My mums H once swore at my DS. I heard him from the kitchen while they were in the garden. I coughed. He swore again 5 minutes later, and I said something. He had a go at me.
These pathetic little men think they have a right to demand you do what they tell you. In fact (and the Stately Homes thread will attest to this, as I live posted from the bedroom waiting for the Police to turn up) I was actually told "Do as you are told"
I told him I was forty fucking five and didn't do what I was told any more, and that he needed to do what I said and LEAVE MY HOME.
they still didn't.
I know you feel in the wrong, this is due to the conditioning you have suffered your entire life, but seriously love, listen to everyone here and think things through, it doesn't make any sense in real life. what he did was wrong, what your mother is doing is wrong. consider this and try to accept it and perhaps you will save yourself the £45/hour it cost me to unravel it :)
Long term though, all jokes aside, you would benefit from some talking therapy.
Im glad you have a normal ExH, and that your dad is there for you. it will help you greatly.