Two ways of going about this, the selfish way, and the kinfolk-care way. The selfish way would keep that knowledge to yourself that way your sister knows you're trustworthy, and may keep some of your secrets, sibling love. The other way would be telling your parents, she's 14 - she shouldn't be drinking anyway, if she was any older maybe it wouldn't matter but it's her health that's getting forgeted with, so you choose.