I, too, grew up around extreme excess of alcohol (and drug use) and violence. I am also a parent.
I am pretty sure, positive infact, that you will not find a single person outside looking in who will say this is in ANY way acceptable, or that you are being unreasonable for wanting to protect your son (and, let's face it, yourself) from this kind of thing happening again.
I also would bet a million pounds that your Mum is carrying a lot of guilt about this: we're Mothers; when our child 'goes off the rails' it's our fault, right?
She needs help too, (as she may well recognise), as one half of a co-dependant relationship. Enabling isn't love and it isn't right. And I suspect she knows this but what else can you do?
I think that's the issue here, isn't it? What can actually be done?
I'm sure you know this but your brother is on a road that doesn't lead anywhere good - you know, all the rockin' destinations such as 'jobless', 'ill', perhaps a detour to 'court' or scenic 'prison' and of course the ever popular 'death' via 'alone'.
At what kind of level is he functioning, usually (and is this the first 'slip' in a while?).
His children need protecting, too, of course. Could be many outcomes of them witnessing this regularly, again, none of them good: learning that this behaviour is ok and normal and so emulated or knowing, even on an instinctual level, it's not ok and the feelings of insecurity that evokes. Children will take on the role of protecting, lying for and parenting their parent. How long before he physically assualts one of them, intentionally or not? Again, another whole can of worms.
At the very least their mother needs to know the full extent of what happened. They thought he was ill? Well, he IS. It's just not a 24 hour stomach bug that will blow over. And his getting well isn't going to be driven by his mother - it MUST come from him and only then is it right to 'stand by him'.
Your son needs positive role models in his life and to be safe (both emotionally and physically). For him to be around this kind of situation is neither. I truly hope your Mum can try and remove the emotion part of what is obviously a very difficult situation for her.
Perhaps you should contact Al-anon as they will have far more experience/advice in a more professional capacity - maybe directing you to services that can help your Mum in the first instance.
Bottom line: you are NOT being unreasonable. Any fracture of the family will not be your fault.