Internet cool guys. Awesome.
The more you know: if you're in an uncomfortable situation where peers are breaking the law and endangering themselves, either remove yourself from the situation or (if you feel comfortable taking such action), inform the authorities. If your sibling is drinking with a few friends in the backyard, a call to the cops is hardly warranted. Inform your parents of the situation, but calling the cops on something like that will exacerbate the situation. I will echo Ravencroft's sentiments earlier in this thread that ages 18-20 are generally alright drinking.
At the end of the day, substance makes stupid people more stupid. Do your research, educate yourself, and party safely. Understand your limits and stick to them. I say this because this community is full of teenagers coming into the age where partying and substance use is commonly experimented with.
Read up, be responsible, set your own limits and stick to them no matter what.