I'm in college; my sister is in high school. Lately, she's been doing some stupid things, drinking and all that comes with it. She tells me about them and expects me to be supportive, but I think she's developing behaviors that are going to ultimately mess her up badly.
It's getting to the point that I want to tell our parents so they can rein her in before something awful happens. She's starting to resent me for always chiding her, and she probably doesn't even tell me all of it anymore because she knows what's coming.
How do I get her back on track and become her sister again?
Answer: You are still and always will be her sister. You can't help her if you, too, get in over your head.
So: She reports her stupid behavior, you chide her, she . . . keeps reporting her stupid behavior. Either her expectations have come unhinged from reality, or you are misreading her expectations.
Helping to rein her in is a way to be supportive, if that's what she wants, and that may well be what she wants. Don't be afraid to ask her, without accusing, why she chooses to tell you this stuff.
Also, don't be afraid to change your responses to her. Apparently, chiding doesn't work. But what happens with questions that force her to think, with sitting back and just letting her talk, with letting her feel for the reins on her own, or with just, "How are you doing?"
Finally, if you sense she's in real danger, don't be afraid to tip off Mom and Dad. It's enough that they know you're concerned; "I think it's important that she feels she can trust me" is the explanation you can give them for not spilling every last bean.
It's enough because it gets the right people into the right roles. Where there's bravado, there's fear, and where there's fear, there are (good) parents - not barely older, doubt-saddled siblings.
True, you might get busted for getting her busted. You also might not be the only one who comes out of it feeling relieved.