I'm so sorry that you're dealing with this. It's really painful. I have often said that it seems like the disease of alcoholism is like dropping a grenade in the middle of a family. It doesn't just tear apart the alcoholic, there's collateral damage too.
It's possible that your mother is feeling guilt and shame. Her defensiveness may be against herself, so she says things openly to try and convince herself (and others, but mostly her) that it isn't her fault. This may be to avoid the feeling of shame. It's a brutal feeling and many of his run from it as fast as we can. Deflect responsibility to anywhere else but ourselves.
She doesn't need any excuse whatsoever to claim that the disease is not her fault. And neither do you. No explanation at all.
You did not cause it, cannot control it, and cannot cure it.
The thing is, the disease will convince the addict in whatever way is possible.
Had a good day? I'll drink to that!
Had a bad day? I'll drink to that!
Sister coming by, feeling uncomfortable? I'll drink to that!
Stubbed my toe and it hurts? Maybe a drink will help.
Feeling anxious? drink
Feeling excited? drink
Pissed at mom? drink
FEELING anything at all? drink
I wonder if there's ONE single circumstance in THIS ENTIRE UNIVERSE that cannot be used to excuse drinking. Whether things are bad or good, up or down, left or right, whatever. Time to drink.
The reason is that there's a disease at play. Not even your brother is at fault for the disease. His only responsibility is to acknowledge the disease is present and to seek help (in my humble opinion - debatable topic!).
you're not alone in your feelings. Keep writing!