How do you tell a friend or a sibling that they need to stop doing something, when they don't listen?

The human will is strong for better or for worse. There is no set way to force someone to stop doing something you think is wrong or hurtful to them. If a person wants to do something, odds are that they are going to do it whether you like it or not.

The best way to handle a situation where someone close to you is doing something you don't approve of is to tell them straight out. And if that doesn't work then you need to go further and tell them the cons of those things or why it bothers you. Being clean cut in your talking to them is important, it shows your trouble and worry with it is real.

If the problem is something bigger, like drugs, nagging them about it probably won't solve or help. If anything it will only make the situation worse. If it is a serious problem then you need to maybe go over their heads. In the question you said sister or friend, and if that's the case then you need to either talk to your parents or your friend's parents. You might be afraid of being seen as a snitch but sometimes that small negative is worth the positive.

Whatever you're referring to might not be as serious as drugs or anything in that realm. Either way, always remember that there are calm and sensible ways of dealing with just about everything life throws at you.