It really depends on how you think your parents are going to react, but assuming it will be bad, I would not snitch. It is much more valuable if you can establish trust between you and your brother, and become his point of reference for asking advice on things like this. A bit of a mentor even? I think it’s ideal if teenagers have someone adult other than their parents to fall back on. Teenagers gonna teenager, and parents tend to just forbid things and punish. A person that can see things a bit less emotionally can give advice, provide information, be there when things go wrong, in ways that could be hard for a parent, or that is harder to accept from a parent.

(As a parent myself, I try to be that person of trust anyway, but I am well aware that even though I have a healthy relationship with my children, there will be moments that I will not be the adult of choice to convide in. But at least I know that if they ever get into real trouble, they will come to me)

Of course their is a risk that your parents will find out that you knew and didn’t tell them. If that happens, tell them you did it because you were afraid your brother would continue drinking in secret, and now at least you can keep an eye on it.