Hi, I'd like to help you with your question.
You have some choices when dealing with your daughter.
First, you can let her stay in your home based on the condition that she seeks help. If she is using right now, it sounds like she might be in denial. As you probably know, that is the toughest stage of dealing with drug use. However, since you are supporting her, she must follow the rules of your house and if the rule is no using, then she has no choice. Also, is she in school or is she working? If she is doing neither, then she must choose one. She cannot stay and freeload from you. These are some of the rules that you need to enforce, if you have not done so already.
If you impose rules and she choses to stay, then she must get treatment. Start with an evaluation to see if she is using and what other issues are going on. Let her know that whatever is recommended by the therapist she must follow. If she does not, then she must leave.
You can also tell her to leave now. This is not the best option, but it lets her know you will not tolerate her behavior. She is an adult now and therefore is responsible. This is the hardest choice, but it does let her be the most responsible.
Unfortunately, you have to draw a line with her behavior. She is not going to respond to anything else but rules and your willingness to stand behind them. If you find that her behavior is becoming too stressful for you, seek out counseling for yourself. You can use the support for yourself and also it will give you a chance to work out the best solution for you and your family. One place you can start is http://www.al-anon.alateen.org/. You may already know about this organization from your sons' experiences, but it never hurts to seek out help again. Also, your local community mental health center is a good place to go for therapy.
There are also some good books for you to try. One is called Don't Let Your Kids Kill You: A Guide for Parents of Drug and Alcohol Addicted Children by Charles Rubin and another is Choices and Consequences: What to Do When a Teenager Uses Alcohol/Drugs by Dick Schaefer. Both of these books can help you gain better insight on how to handle your daughter's situation.
I hope this has helped you,