I'm going to tell the brother of my long distance gf that she's been drinking and driving
Wow are you serious?
He needs to know the truth.
You definitely should your gf's brother know that she's drinking and driving. It's good that you're looking out for her.
I'm not sure what happens if he tells her.
I am happy you told him, maybe he can get her the help she needs. Drinking and driving is so dangerous she could hurt or kill someone by accident. You did the right thing and never second guess that.