Am I the only one here who thinks there may be a whole slew of overreaction going around? A 19 year old who can still get by on 2 airplane bottles of any liquor is far from someone who has a problem. Heck, I'd go so far as to say that most 19yo kids have had far more experience under their belt than that. If you want to tell your dad, do it knowing that you're doing it to save your own skin. This talk of saving your brother from walking down the wrong path sounds like a lot of moral posturing and overly melodramatic judgment to me. I was a hellion in high school, but by the time I got to college, most of it was out of my system, and I continue to have a healthy and balanced relationship with intoxicating substances (if you don't believe this is possible, that's another thread entirely). Some of my friends on the other hand, came from entirely sheltered backgrounds, and crashed and burned pretty hard with all their new freedom. People learn best from their own mistakes. Let your bro figure it out for himself. Pleeeeeze, 2 airplane bottles is a far cry from a drinking problem.
I for one, have to agree with the others who suggested you speak to your brother before going to your dad. Giving him the chance to fess up himself absolves you of possibly having to rat him out, and lets him make his own decisions. Talk it out with him, don't go behind his back and snitch. I for one, would never have sold out my brother unless it was absolutely necessary for his health and safety. 2 airplane bottles sounds more like a sheltered but curious kid than someone on a full on bender of self destruction.