First off, we're obviously only really hearing the one guy's side of the story, and he's not exactly going to admit if it was more then just providing alchohol to kids and letting them party.
But if his version of events is even vaguely accurate, I think that's a ridiculous amount of jail time. First off, where did the girl come from? Someone just rubbed a genie bottle? Presumably, she's a friend of one or more of the guys there and presumably also agreed to come to this party and consume alchohol. I guess what I'm getting at is that while this particular instance occured with the older guy involved, it's not unlikely that this exact thing wouldn't have occured even if he hadn't.
Let's accept the fact that teenagers are going to obtain alchohol and/or drugs and use them to party. I did it. Everyone I know did it. Probably everone reading this did it. And in most circles, it's "safer" if said party occurs as a result of an older sibling/friend obtaining alchohol for the teenagers and sticking them in a specific location (like someone's home), so that at least they aren't getting drunk in some random location and then wandering around the streets and/or driving while still intoxicated. Having the older sibling provide them with alchohol and a location to safely consume said alchohol is generall better then what they'd likely get into otherwise.
Wasn't there a big deal with those "contacts" between parents and teens a while back? I recall there being a push to encourage parents to provide safe places for teenagers to party, understanding that there would be alchohol, but also understanding that there's be no driving once the kids arrived. Sort of a "don't ask, don't tell" approach, that is pretty darn intelligent and mature. I don't see what this older brother did as any different. He provided a group of kids with alchohol and a safe place to consume it. Should he also be responsible for the actions those teens take while in that location? Sure. In a technical legal sense, yes. But I think there's a whole lot of legal argument that he didn't cause what happened to that girl to happen. Unless he was directly involved in that, then I'm having a hard time seeing it. She chose to go to that party. She chose to drink. She was provided a safe location. The exact choices she made could have ended her up passed out in a back alley behind a rave somewhere, with potentially far worse done to her, then what happened at this party. Can you really hold the adult responsible for that?
I'm just not seeing it. Sure. Maybe a slap on the wrist. Hit him with a misdemeanor "contributing to the deliquency of a minor" charge or something. But 6 years in prison? That's ridiculous...