Jaime.. First I like to say hi and welcome to 4WT.
I feel for you for your in a difficult situation, you want the best for your kid brother and you don't want to him to mistrust you for your close to your parents and you want him to confide in you. Without him thinking that you will police him and tattle on him to your parents.
He is only 16 yrs. old a diffucult age to be. Lot's of peer pressure etc. We have all been there done that. He will not listen to reason at the moment. If I were you and he sounds like he trusts you with what he confides in you. I would sit down and have a talk with him. The dangers of drinking, drugs ,sex and smoking at this age. Sometimes you have to repeat the things that they don't want to hear, I find as mom it does get absorbed . They may not agree and try to go with the in crowd.
I think since you are close to your parents, you should confide in them. It is their place to step in with whom your brother is hanging out with. Not yours, you should not be the bad guy. Besides he probably will not listen to you anyway.When you have this discussion with your parents, tell them please do not involve me , and that you heard nothing from me. It's good that your not to much older and probably know his hang outs and friends. Definetly keep an eye on him.
My children when they were teenagers , if one didn't like what the other did or heard something that they didn't like. They would tell me and I would handle it...
It's hard but your a good sister and your just care about him. He will realize later on in life..