I did the same thing. I was at the drs every few weeks, then I would call and say my wisdom tooth was infected or I need my script early because we are going on vacation. You do and say whatever to get them. My Dr called me out on it because its obvious I am addicted. What scared me is the forum where people are so sick from overdose and going through withdrawl, show her those people, its really sad. I will never forget a woman said she didnt even care about her kids, they could just run around the house and do whatever while she laid in bed popping pills. She didnt care about anything but getting more pills. Hope you can get through to her, she is still so young and I remember that age I thought I was invincible.
i think you should do some straight talking with your sister, apart from being addicted to this drug, she will do her stomache no good what so ever, ulcers etc , talk to her and tell her you will inform your parents if you dont get some straight answers, being addicted to this is no picnic, and she may need help, or to realise the seriousness of this, best wishes,