Hi. I am sorry for your trouble. There is actually help here but you MUST be willing to be brave and ask for it. I am in the U.S. and I think calling DCFS is a good idea. A friend I once had got a lot of help for a twelve year old boy by doing so in Illinois. He was breaking things, violent, mentally I'll and a danger to his sister so he was taken to a residential treatment center. The family finally had peace and he got treatment. I don't know the final outcome because the family moved, but they were doing better and son was allowed visits home unless he acted out, then they took him back or the police did. He was adopted and had a form of schizophrenia that he inherited from birth mother and had an attraction to his eight year old sister and he admitted it so he was not safe at home.
I can.tell you a few things I myself would have done or considered. First of all, it is illegal for a 21 year old to touch a fifteen year old and I would have turned him into the police for statutory rape. Daughter may not be scared of police (yet) but she can't control the law.
And at 15, especially with serious drug problems, I would call CPS (your idea) asking for help, rehab or a residential treatment center and I wouldn't care how angry they got. They need serious help and you can't do that. You aren't able to....you are not a professional. You can't force them to do stay sober but others can in a residential environment. You need a break from them too or you will get sick and that will help nobody. You need to take care of yourself. It sounds like they need to be off the streets too.
Is there a father? Other kids at home that they can influence or hurt? Are they dangerous to you or do they destroy your property? Steal? Have you ever pressed charges? That is one way to get some activity going, maybe a rehab or residential treatment. Or it could be juvy hall but such young kids may get a better choice, but you have to make it happen. Or nothing will change. By 18 your only option will be live terrified or make them leave. It is better to do what you can now.
You need to be at peace and feel safe under your own roof.
At the very least you can go to Al Anon to talk to other people in the same boat to find out what they did and get support. I did this. It really helped me although I mostly just listened and cried for a few months but people reached out to me. I had no idea what to do or that I had a right to take care of myself.
For sure first turn in m the slimebag 21 year old who is sick enough to date a 15 year old. This would have happened with me right away.
Love and light. Others will come along. Stand strong!You can do this.