I think the poo is gonna hit the fan
 

My SIL (AH sister) is coming over tonight to talk to him about his drinking and taking anti-depressants. Very nervous here.

Why are you nervous? What do you think is going to happen?

I don't know, lots of yelling. Him taking off in the car drunk. No telling.

If he takes off in the car drunk, you can call the police. I would.

Yes its your moral responsibility with your neighbors and general population. He doesn't have to know you were the one who called.