I want to reiterate Gotta's words: this is not your fault.
I grew up with an alcoholic father and my mother would tell us that he was just "tired" when he would start fights with her and verbally and emotionally abuse us all. So for the longest time I thought it was normal behavior, only to subconsciously tell myself that my dad was unhappy and it was my fault.
This is his choice. It's terrible that others have to pay for it, too but nonetheless it's his battle to fight.
If it were me I would tell my mom all I had witnessed. In my opinion, staying with an active drinker is toxic too as it's enabling, however inadvertent.