No offence intended at all, but I think your parents are being over-protective because you're not acting 18. You sound like my 13 year old sister (again, not trying to offend, she's my sis and I love her). Stop being so possessive of your mutual friend, and instead of bitching about the other girl, have a word with her. If you calm down and speak to her in a mature way, then she'll listen and stop with the sarky comments. But while you're acting like this, you're sinking to her level and she's unlikely to listen to you. Plus, when you continue with this kind of attitude, you can't really accuse her of being childish or whatever (which she is, I totally agree with you there), because that would be very hypocritical.
I also think if your parents gave you some space and responsibility, you would learn to handle this kind of thing with no problem. Tell your "friend" to grow up or piss off, and let your other friend decide for herself. And get a new best friend!