Forgive me for crashing your forum, ladies
This sounds like a risk-management nightmare.
Does AST have anything to say about the issue of underage drinking? In AEPhi we are expected to abide by all state, local, and university laws/rules/regulations regarding alcohol. Violators can be called to standards. If AST has a similar rule it might be time to give your sisters a nice friendly reminder of that fact. It doesn't have to be a hard and fast "thou shalt not drink" - it can be closer to "be careful if you do drink".
You could also try making sure there's less alcohol around the house. If it's not there, they can't drink it.