I Need to Vent

My mom knows alcoholism runs on both sides of the family, but she is not only letting my sister get away with drinking underage (the brat turned 18 last month), but is telling her how to get away with having it in her car. Her response when I said she was being irresponsible?
"They're going to drink, anyway, so no point in trying to stop them."
She's also going to deny that princess and her friends got the wine coolers from our fridge, so there's that as well...