The more people (like you, obviously) are against it, the more she is going to want to do it. You're only 19, I'm sure you remember exactly what it was like when you were 16 and wanted to rebel. Chances are, if you left them well enough alone, the whole thing would fizzle out on its own - but if you try and fight it, she'll go through with it just to prove you wrong
btw, dont minors under the age of 18 need their parents approval? Since someone under 18 can't legally enter into a binding contract, without approval of their parents/guardians? Correct me if I'm wrong here.