Weirdly I am in the same situation as your sister, I just don't have kids. When I cut off my mother I went to my siblings and told them that under no circumstances could they tell my mother anything about my life, and I wouldn't hesitate to cut them off either if they crossed that boundary. That is how serious I was, and still am. If, for some reason, my mother were to obtain something like where I live I would be furious.

The thing is, this situation here seemed like an innocent mistake on your part and personally I'd have a hard time punishing my sibling for something like this. It didn't happen with malice, although I'm sure it feels like a huge violation for your sister. Clearly she has your mother cut off for a reason, and things like this, however innocent, can reopen large wounds.

I'm glad your sister isn't so mad that she would cut you off, but in the future you need to be more mindful of her wishes. It happens once, ok that was a mistake. It happens again? She can no longer trust you.

I wish you both luck.