How can I talk to my sister about her drug problem without her getting defensive and/or leaving?

I think you should try and be as empathetic and considerate as you can, it would be nice to attempt to make her feel safe and understood.

I think coming from a harm minimisation frame of mind can help (which can be extremely hard when it is a loved one) but not condemning or condoning the drug problem, but rather exploring how to reduce the harm associated with their behaviour/problems. Also motivational interviewing is a great technique for this, which would go far over the 1000 word limit to explain fully (and not an expert, but I use it a lot with my clients), it's about establishing a collaborative non-judgemental conversation style for strengthening the persons own motivation and commitment to change. It's about finding that ambivalence (if it's there of course, it usually is we can become stuck in ambivalence for a very long time), if not a pro and con list (but a tad more subtle) can help. Also, with a non-judgemental attitude, you can try using a directive style that argues for change with, in this case a sister who has a drug problem, trying to find out why she enjoys using drugs but also why she doesn't which can create awareness of or the ambivalence itself. Focusing on that ambivalence can get the person themselves to argue against the behaviour themselves by creating and opposing view, in doing so a person is usually much more persuaded by their own words and hearing them out loud. It is then you can try and support them in what they want to do, just respect them, their decisions and their feelings/thoughts.

Drug addicts don't think they have a problem.. You can't help someone who doesn't want to help themselves. I know from experience.

First of all, avoid accusations and shaming comments. Instead, use "I" statements to let her know how her problem affects you and what you expect her to do. For example: "I've seen you using drugs 5 times this week. It makes me feel afraid for your well-being. I would like to help you. I hope you can tell me how I can do that."

Don't approach her about the issue. Just be a better sister, but be her friend most of all. Maybe you can help her once she lets you in. Good luck. :-)

If you try to talk to an addict about their problem they very well might get defensive and leave. It's sad but it can't always be helped. Certain ways of communicating might be more effective if they are open to it. Pick a time that isn't extra stressful and make it personal. Don't blame or accuse. Use those "I feel statements". Let them know that you care and love them so much and would support through recovery but that you aren't going to enable them in their destruction.

Let her know that you're not getting at her; you're concerned. You want to make sure that she's safe and when she's gone, you're worried about her. You just want to know that she's not going to disappear one day and never come back. Let her know that you're there for her and you always will be.

Explain that you're there to help her and that you understand what she is going through and how you will all help her get through the problem.

There is no good way to confront someone with a problem. The best way might be to tell her what you want to in a neutral tone with no hidden remarks in your words. If she does get defensive remind her that you are only trying to help her. If she tries to leave, that is her decision.

I think you should try to be understanding and make her feel safe and also try and be considerate as you can