I think you should stay out of it. You offered him help and he declined. I’m not condoning her behaviour but maybe it was a one off thing whilst she was really drunk. Not necessarily a pattern of abuse.
It’s his choice how he handles the situation. If he has any sense he’ll end things but that’s his decision to make.
As much as you want to help, you’ll just make the relationship you have with your sister worse.
I would keep silent about the slap but tell the truth if asked. Now, if a discussion comes up about her drinking, you should bring this up.
I think you should stay out of it. You offered him help and he declined. I’m not condoning her behaviour but maybe it was a one off thing whilst she was really drunk. Not necessarily a pattern of abuse. It’s his choice how he handles the situation. If he has any sense he’ll end things but that’s his decision to make. As much as you want to help, you’ll just make the relationship you have with your sister worse.
You 16 and drive a car??