I know this is not my place to tell you what to do, but you did ask for advice. Your post has stayed with me for a number of days and so that is why I am replying. I think it is irresponsible of you to be telling your niece to do things that 1) are illegal (assuming you/she are in the US), and 2) are against the desires of her parents. I think you are setting up a very bad dynamic between her and her parents. She is not living with you and you are not her legal guardian. If she gets in trouble for drinking or doing drugs you do not have to deal with the fallout. Her father,being a cop, would not have to only deal with the legal fallout for his daughter, but also any influence it has on his career.

Also, as long as your niece has a good support system and understands the dangers of drinking, etc. And is provided a realistic view of how to take care of herself she should not get into trouble when she goes to college. There are a large number of students who go to college without drinking and/or doing drugs who then subsequently do not get into trouble with them in college. If you worry about your niece getting into trouble when she is in college, I think a better way to help her would be to talk to her about the pressures she will face and help her find ways to stand up to the pressures to binge drink, make bad decisions about drinking and drugs. This would be better for her in the long run than suggesting to her to do them now. Also, in the US if you have a drug arrest you are excluded from all federal financial aid. This means that a lot more money would be coming from out of pocket to pay for college and/or from other lenders. So these decisions now that you are pressing on your niece have long term consequences if she were to get into trouble.