Yes. As others have said, absolutely in the case of your father, it was abuse. He was old enough to absolutely comprehend what was going on. To not only witness it and not stop it but to participate in it seems very wrong. Maybe the hope was that you'd learn to fight back but it seems very messed up to me. It sounds as if both seemed to enjoy picking on you.
Regarding your brother, that's more tricky. Children can be incredibly cruel and maybe we don't have full perspective/ empathy at that age.
I sort of struggle really. I had a very mild version happen to me growing up. Someone also tried to strangle me one time- in front of a friend and, held my head under the water another. I played dead though and it scared the shit out of them. I was rather pleased with myself for that.
It's hard to entirely call it abuse but in a way, it still was. This person was trying to test boundaries all the time. Just how dominant they could be and get away with it. I believe them to be a narcissist and with a whole bunch of other stuff, I would describe what I went through as typical of narcissistic abuse.
My experience was more psychological though. No where near as physically brutal as yours. Plus, not so blantant in front of my parents. I think that is the most troubling part of your experience.
What is your relationship like with them now- may I ask? Did your brother eventually mature or, is he still a bully?