I'm not sure Brian's silence is a confirmation of Joan's claims here, considering the situation he might not want to antagonize her further or maybe he's heard the speech before and doesn't see the point or he could just be speechless; anyway, I didn't get the impression that he agrees with her view of the past.

Concerning Joan's martyrdom, from what we can see I'd say it consists mostly of whining about how bad she is and the rest of the world not appreciating her - her husband never being home and calling her "the warden", her son neglecting her, no one coming to visit her, I'm sure she has some complains about Claire as well - it's more that she likes to see herself in the martyr role than actually "sacrificing" for something. (Maybe she sees having Brian in the first place as a sacrifice.) I'm not saying she never stopped Jack from hitting Brian, but I think that she gave it up before Brian was really old enough to remember it in favor of drowning her sorrows in prayer and wine.

I agree with you on the physical abuse though. Jack probably didn't put much effort into holding back when he was angry, but I think for the most part the abuse consisted of absence, desinterest, verbal put-downs and a general lack of love, warmth and security from both his parents.