UPDATE: My (22F) sister (25F) is still addicted to drugs

Original post: https://www.reddit.com/r/relationships/comments/6m72mb/me_22_f_with_my_sister_25_f_shes_addicted_to_drugs/

Hi again, I just wanted to thank everyone for their advice last time and update you all on what's happened. Sorry that it is not very positive and please believe that I tried my best.

After my last post I went overseas to volunteer for three weeks and so didn't see my sister Kate much, I thought about leaving her a letter but didn't. Before I left I was at my parent's house, where she lives, but as usual she was out partying. When I arrived home she invited her new boyfriend (20M) over and hung out with him instead. I was a little bit dejected.

I'm back living with my parents for 2 weeks whilst I wait for my new flat to be ready and today the whole situation blew up. A letter arrived in the post from the mum of Kate's ex-boyfriend, which asked my mum to call so they could talk about Kate. It didn't say anything specific but I'm assuming it's about her drug addiction which led to their breakup.

I called Kate to ask when she'd be back and I was pretty sure I could hear in the way she was speaking that she was on coke and we got into an argument about what constitutes addiction/using too much (am I totally off the mark or is regularly doing large quantities of cocaine 3-4 times a week over prolonged periods bad? I'm almost certain that it is a terrible idea health wise? Not to mention how much it costs.) I hang up. Which was definitely immature of me.

When she finally arrives home my mum can tell there's an atmosphere and calls me down to ask what's going on between us. I'm still too cowardly to say anything and Kate is smirking and saying things along the lines of "I don't have a problem with OP. Different people just have different hobbies." (again I don't consider using cocaine and other drugs as a hobby but I might be way off????) I was really annoyed because it felt as though she was daring me to tell our mum about her drug use, knowing I wouldn't because I don't want her to get in trouble. It felt really manipulative and horrible which is a feature I haven't seen of her personality before. I avoided the question and went back to my room.

She left to go to her boyfriend's again and I messaged her to let her know that I wouldn't be talking to her or having any relationship with her until she quit drugs. I broke down in tears and told my mum everything. Turns out that my mum had a vague idea, through seeing messages pop up on her phone, but she doesn't know what to do. She told me to take a time out for a bit and not trouble myself with my sister's life problems. She seemed pretty resigned to it all, just like the rest of us, racking her brain for where she went wrong and what she could do. Short of kicking my sister out (which wouldn't get her to quit and which my mum isn't willing to do) there's nothing she can think of.

It's all quit depressing really, the replies to my last post made me realise I couldn't force her to change, as much as I want to. She was my best friend and I just really can't watch as she ruins her life and kills herself. My other sister is implying I've overreacted and that her drug use wasn't that serious and she would have gotten over it... Which is a bit of a bummer since we had always been on the same page about Kate's addiction. I'm going to stay with my boyfriend for a bit, it's my birthday next week so we are going to have some drinks and relax. I feel really guilty cutting her out and don't know if it was the right decision but for now I just can't deal with the stress and upset so I had to act selfishly. If she sorts herself out, or takes steps towards it, I would welcome her back in a second.

TL;DR: my sister is still addicted to drugs, I'm no longer talking to her, I told my mum.