I am so sorry that you are living through this. You have GOT to get the authorities involved. I don't think finding a diagnosis will do a whole lot right now. He could likely spin a sob story and twist some therapist or psychiatrist (therapist and psychiatrist respectively around to the point where you are told that he is the victim. I don't know if he had Reactive Attachment Disorder (RAD) from severe neglect/abuse before age three, if he is a sociopath and/or if he had conduct disorder or if this is largely the drugs. NO ONE can tell you any of this until he is off of the drugs. They make ANY diagnosis impossible an adding psychiatric medications on top of drugs is pointless for the most part.
You MUST, sadly, make the safety of the other children the highest priority. Unsafe behavior equals a call to the cops. Make very sure you have a cell phone on your person at all times and practice dialing 911 without looking. Call the police non emergency line to see if your area will allow you to register your address to your cell number. I know my area does and we are not a large city so the technology for that isn't hard to get. This lets the police know where to come if you cannot talk on the phone when you need to call.
Also practice calling for help with each and every other child who lives in your home. Let them know that if they call 911 because he has hurt or threatened them, that you will back them up and not the 15yo. They NEED this. You also must not EVER leave them home with him with-o an adult. It is way too dangerous.
I know you think you have a pretty good handle on things and know most of what he is doing. From first hand experience and I only had a total of 3 kids, I can PROMISE that he has done and is doing FAR more than you are aware of. You will only discover this stuff after you have gotten him out of the house and the other kids are sure he isn't coming back. I very much have been n somewhat similar situation as a parent, and I was TOTALLY in a similar situation as a sister.
My bro is a difficult child. My parents didn't acknowledge 1/10th of his problems/behaviors. NOW they want to know why I didn't tell them. I didn't tell a lot because I was afraid of him. They didn't believe the things I did tell them, so I really didn't trust them to do anything. I was more afraid of what he would do if I told than certain that they would keep me safe from him. At age 12 made a plan to kill him if he did certain things ever again, or tried other things in the future. I was totally serious and my intent with the plan was not to warn or scare him, it was to end his life. I am now amazed I had the strength to make the plan, shocked at how well it most likely would have worked, and deeply saddened that I felt so alone and unprotected and unable to find help that I had to make the plan. I wanted my kids to never feel that way. I wasn't as successful as I wanted to be. My kids told me a LOT more, but until he was out of the house I didn't know nearly enough.
You NEED to start calling the cops every time he is out of line. INSIST on pressing charges. If he hurts one of the other kids, or you, physically, flat out refuse to allow him to come back. Tell them that you are afraid of him. Also, you MUST get child protection involved NOW. Make a report that he is sexually abusing his sister and has tried to peep on her friends, and that he has also hurt his siblings and you need HELP. Yes, they could take all the kids away. As long as you are doing all you can, you put locks on doors for each child's safety, you give the youngest one of those personal alarms to use if he does ANYTHING (they run five or less at Walmart and radio shack, dollar tree sells them for a buck) that scares her, heck give each of the kids one to protect them.
CPS will also want to see a written safety plan. Who does what, goes where, etc... when difficult child does something unsafe. I would suggest the other kids all go into a room with a land-line together and have calling 911 be the first thing they do when they feel unsafe. Better a few calls that were not so serious than no call where something awful happens.
I know it is hard. He very well may need help with something other than drug abuse issues. Sadly at his age you pretty much are helpless until he wants help, which sure doesn't seem likely.
You MUST protect the other kids, and know that getting him into a position where he cannot hurt anyone really is in his best interests. It may feel mean to call the cops, but isn't it better to teach him now what happens when he hurts someone than to wait until he puts someone in the hospital or morgue and the only option for his either death row or life in prison? At his age most states WILL try him as an adult.
The peeping on the youngest is very serious. This is one of the steps that leads to becoming a sexual predator, and it isn't the bottom step. Get your 10yo to someone capable of doing a proper interview over this because if she has told you this, he has almost certainly gone much further. He has threatened her to keep her from talking,and the right person asking will figure out what has gone on and if it has gone farther. They can help you find the right help for her, which she very very much needs. You should NOTNOTNOTNOTNOT ask her about what all he has done. Asked in the right way, the truth will come out, but the more she is asked by people not trained in asking these questions, the more likely it is that the wrong answers will come out. NOT NOT NOT because she wants to lie, but because she is a kid and wants to please. Regardless of the issue, if you ask the same question several times, kids will change their answers to what they think you want rather than sticking with the truth. This is NOT saying she is a liar, it is saying she is a child and human. Actually, lots of adults also do this, it is why there are so many false confessions. So please do NOT ask her about the peeping, but take her to a professional who is properly trained and can let you know how much you have to worry about and how to help her heal.
From the parent of one dangerous child to another, STOP allowing your children to bring friends home. For as long as he is in your home, you cannot invite other children into your home. It is DANGEROUS. What, exactly, do you think would happen if he harmed or molested or raped a guest in your home? Do you have ANY clue what that would do to your other kids? They would ALL be ostracized. VERY few other parents would allow your kids to play with theirs in public, and as far as coming to your home or going to theirs? It won't happen after your son harms a friend of your family. I have seen it happen. We were blessed in that a family that lived close to us for the worst years was my daughter's best friend. We are close to the family and I confided in them because I didn't want them to think their child couldn't come over because we didn't like her. It was for her safety. They responded in a way very few would - they gave my 2nd grader a key and said to come over any time, day or night, no matter what. I am VERY sure that if her friend had been harmed by my difficult child at our home the response would have been very very different. I saw it happen in my community a couple of years ago, and it was ugly and sad.
You don't want that to happen. Y Our kids are enduring enough with-o having all that heaped on. Do what you have to. Keep calling the cops and wanting to press charges until they put your son somewhere he will get help and not be able to hurt anyone. If you don't, the things he is doing with peeping and physical abuse can and will be used to take all of your children from you for their own safety. That is NOT what you want.