Your husband said that you 'couldn't get your head round' how far across the line you'd gone. Or words to that effect.
He can't get his head around the fact that these people severely abused you as a child and teenager. Maybe that's partly because you are only just starting to acknowledge this yourself. The behaviour you described that they inflicted on you is utterly horrific. But although you know and have said what they did, you didn't think of it as abuse, and say that your sister was the 'good' one which implies you thought you were the 'bad' one. So you also hadn't fully got your head round this fact - your parents abused you horribly. This incident is a consequence of that.
I understand you have craved their approval. I understand you wanted your children to have grandparents. BUT - not these grandparents. They were not good enough as parents to you, not by a long chalk. Their approval is never, ever, ever going to happen. So do not waste your energy on that. They are vicious nasty people. And they have just shown you once and for all that it is of absolutely no benefit to you or your dcs to have these people in their lives. I understand that you want your dcs to just have a normal family with gps, but that is not the reality. It is your absolute duty to protect your dcs from their harm.
Your intense anger issues are a direct result of being totally powerless in the face of this hideous childhood abuse. And yes, you need to go and seek help to sort all this, proper psychotherapy. Not only how to manage your anger, but also to acknowledge fully the abuse you have suffered and how to protect yourself and your dc from your twisted and cruel parents. And somewhere in there, your husband needs to be educated and brought up to speed. If he stays on the side of the abusers after it's explained fully to him, then you'll be able to make a decision about whether he is a suitable life partner for you.
You have done well so far to survive this, to break the cycle of abusing less powerful people. But now this is the catalyst for you to get some serious professional help.
Everyone will be outraged by the slap to your mother of course because violence is wrong, shocking and not the answer. Hopefully though the result will be a total breakdown of your relationship with them. Your sister's 'golden child' position is what makes her side with them, otherwise that position is threatened and that feels unsafe for her. Horrible dynamics, but true.
Your own violence is when your own 'child' (the abused girl you were) is triggered and comes to the fore. Only that horribly abused girl is now in an adult body and capable of fighting back!
I know you feel awful about what happened and your husband is trying to make sure you feel awful too, but think how you would feel about yourself if you had allowed her to continue terrifying, terrorising and violently abusing your son. Even worse. Your mothering instincts are in very good shape. Which is more than can be said for your mother.
That is 40 years of justified fury, hurt and defences rising up and taking over. Your fucking horrible mother caused this. Obviously the violence is unacceptable. But that's one slap versus teaming up and dragging a naked teenager (their own child) out of the shower to be held down and beaten, and all that other violence that came from them. So yes it wasn't ideal, but my god she was already in the process of carrying out an assault herself! And she wouldn't be stopped.
Best of luck 