Heyy :) From reading this, I can definitely tell you that you're not coming off as a hypocrite. More-so, you seem like you're trying to protect and shelter Maria and not get her into the habits which you've gotten yourself into. This isn't a bad thing at all, and you shouldn't 'stop' doing this by any means. Perhaps rather than telling the other girls not to bring her, the best thing you could have done was talk to her yourself. It would open up to some conversation, and in turn, lead you both into a discussion. Could it stop her from wanting to go to those things? Maybe, but it could also encourage her and have the exact opposite effect that was intended. This is pretty much the obvious thought, but talking to her yourself is always going to be the best way of getting your point across thoroughly.
Sure it's going to be hard, but it's better than having her go through the exact same things that you're trying to prevent her from doing. They are dangerous, as most parties tend to be. You could try to lead by example, sure, but that will only drag you away from the situations. Perhaps this is the best thing for you. Who really knows except yourself. The thing is, you have to expect her to go out and do these things. She's a teenager, and at one point in her life she has to be shown these types of things. We all do. Maybe she won't like it, or maybe she'll come to the realization of how 'bad' they actually are. She, like most people, has to learn from the past and her mistakes.
The only thing that you can really do is to allow her to experience it like everyone else, and to open conversation with her. :) Don't be afraid to discuss things with people, and don't be afraid to let them do things like that. She's not going to get mad at you; likewise, how you wouldn't get mad at her for talking to you about something. The whole party scene with her there made you uncomfortable, yea, but sometimes we have to try and get over these things in order to just allow ourselves to stay cool and calm. You're her friend, and to let this upset and make you uncomfortable is a strange thought. You should try to think of the positives of her being there, rather the negatives.
Personally, I would play the role of both sister and friend. Don't be discouraged to show her your feelings from both perspectives. Or perhaps mend them together into one descriptive thought. She's your sister, your friend, and someone you care about. Show her this, and explain the bad aspects of partying. She will take this into account, but she's going to do what she wants in the end. You should still talk to her no matter what, but don't get too upset if she doesn't listen fully. :) Also, to alleviate the tension between you and your friends, talk to them. Ask them what's wrong, and again, communicate with them your concerns. :) Everything will turn alright in the end, as long as you keep cool. :P Good luck, and just be honest.
~Wayne