How should I respond to my morally corrupt sister?

My 20 year old sister is morally destitute. She is an unapolegetic shoplifter. Her justifications amount to things like: "My shoplifting is not an addiction because I can stop any time I want to," "everyone does it," "you have to lie and cheat to keep up with the competition," "companies account for shoplifters in their business plans so they mark prices higher to compensate for it," "I'd never steal from a friend," "I need to steal while I look young and can get away with it because no one suspects me," etc. Over the years she has stolen hundreds if not thousands of dollars from our parents, too. And justifies this by saying that they're wealthy and don't even notice because she does it in small increments. She lies and cheats frequency. She's accepted money in return for writing a paper for a friend and says of course she would do it again if some one were willing to pay her. She knows what she does is "wrong," but maintains that this is better than not knowing, at least. (That makes no sense, I know.) I also just found out that she's selling marijuana because, as she says, she needs a way to support her expensive taste in clothes and makeup. (Stealing these things directly and the money with which to buy them doesn't cut it, apparently.) She acknowledges that this is, on some level, "wrong," but does not care. She has no integrity or moral conscience. She's my sister and despite my horror at these behaviors and the cavalier attitude with which she knowingly does wrong, but she laughs this off and doesn't care! She does not respond to reason. She is vain, ignorant, and idle. Part of me wants to help her and try to talk sense into her because I care about her and want her to be a healthy person and not have a miserable life, another part of me wants to forget it and let her ruin herself. But I don't want to stand by and watch that happen, and I also know that there's only so much I can do to really help her. What is the rational thing to do?