You mention AA, but you don't mention whether your Mom attends Alanon. It would be helpful for her and for you. It's focus is on the relatives and friends of alcoholics and provides you with tools to help you cope.
I should point out that I have an active AH so my answer is no doubt flavoured by that.
I sense that keeping your father's secret of drinking feels deceitful to you. I would tell your mother for that reason. I don't think you are telling her something she doesn't already know or suspect. I don't think you are betraying your father. His secret drinking is going to come to light sooner or later, you just may be affecting the timing.
Because an alcoholic wants to protect the drinking, your father may be upset with you. When your father finds recovery (not just abstinence), he will understand why you shared.
If I was your mother and discovered that you had kept your father's secret, initially I would feel hurt. That would quickly be overcome with worry for you, that you have/are developing codependent behaviours and all that that entails.
I've heard that you are only as sick as your secrets. For me, this was very true especially when my AH's secret drinking was becoming the "family disease". Maybe sharing with your mom, and creating a family conversation might bring everything into the open. But please, do not count on this happening. If you bring the secret into the open, just leave it there and try not to have any expectations of what will happen next. And if what happens next is chaos and accusations etc., that is not your fault. That belongs to the person who has the bad reaction.
Just another take on your situation. In the end, do what you feel most comfortable with while realizing that none of the options will feel "good".
Wishing you and your parents well.
p.s. I don't think you are meddling.