You said she is a good kid but drinks. I assume this means she is hoping to please you and get along in most ways. have you tried telling her you are concerned and asking if there is anything you could do to help her out? Also, there is a great online magazine - www.teenink.com
written for 13-19 year olds and contributed to by teenagers. Might be a good resource. You could even take a look and see if it gave you any ideas.
I will say that I did my share of drinking as a teenager and while I made bad decisions and have many regrets, I understand that those were MY choices. Looking back I do wish I had done things differently but in order to do that I would have had to have a different upbringing. One that taught me more about respecting myself and valuing my body. Instead of learning the lessons young, I learned them the hard way when I got older. The point is though, I did learn them. If your step daughter is checking in that is definitely one thing in your favor. It sounds like she has had an unstable existence until now and is not used to being able to count on anything. You can set conditions of her living with you. Regular family counseling, curfews, check-ins, no drinking and driving (very important!). I am of the opinion that counseling doesn't work unless the whole family goes. Actually what might work better is family mediation. That is more learning about how to respect each others needs and listen to what each other are saying. It is good skills for everyone.
However, I know people will condemn me for this, I might condemn myself someday . . . but, the reality is that most kids experiment with drinking before 21 and it is a legal construct that makes it wrong for an 18 year old to drink, not a moral construct. 18 year olds can vote and serve their country and die at war. I think many of the problems we have in the 18-21 set have to do with not teaching them about drinking responsibly, turning it into an act of rebellion and not providing safe avenues for that age group for the purpose of their exploration. I think the gap bewteen 18-21 where we expect adult behavior from kids with out granting adult priveledges is ridiculous. I think it should be when you are an adult you are an adult. If you can't drink at 18 you shouldn't be able to vote, or serve in the military or be able to get a phone in your name. The age gap seems arbitrary to kids because it is.
That being said, you are free to voice your concern, disapproval, make ground rules for living with you, tell her you don't like the fact that she is drinking. You don't have to provide her with alcohol or make it easy for her to live a drinking lifestyle. She can then make her own decisions about whether or not she will continue to live with you and abide by your rules, etc. If you make your concerns known, then whatever happens now, she will thank you for caring later.