I am sorry that you are having so many problems. I would NOT agree to MST as what will fix everything. I don't know if the diagnosis's are correct or not, but it sounds like you have done everything you possible can and it is hurting your other child to have him there. Be aware, and make them aware, that he is going to be bigger and stronger than you in the very near future, if he isn't already. This means that youa re going to be in danger from him if he has no empathy. When he cannot have/get/do what he wants he is going to hurt you and possible hurt you to the point that you are permanently harmed and/or killed. This can also happen to your other child. Push HER needs into their faces. It sounds very much like keeping him home is dangerous and may endanger her and your lives. What is CPS going to do if you have asked and begged for help and he kills her or you? Who is responsible then if their MST therapy hasn't worked?
I have seen other parents here trying to get services the way you are. Regardless of what they promise, there will be a waiting period before it starts. A couple of weeks, maybe a couple of months. In that time he will be home iwth you and you will still be dealing with his problems alone. From your other posts it sounds like your instincts are screaming at you that it is not safe for him to be at home.
If this is the case, do NOT give in to them. The biggest mistakes that most of us make as parents happen when our instincts are saying one thing and the "experts" are saying another. We doubt ourselves because they are the "experts" and "know more than I do" and we go along with them. I am begging you to NOT fall into that trap. Yes, they ARE experts. But they are experts in the system and in getting parents to go along. They are NOT NOT NOT experts in your son, his problems, what has already failed to help, your daughter, what she needs, what he has done that has hurt her, and in you, what you need, and how to keep your family safe. YOU are the expert in those things. YOU. So go with your instincts if they say this is all just empty promises or that you have already been there done that and it hasn't worked.
Above all, insist on what will keep everyone SAFE and do NOT let them put your daughter's needs and your needs below your son's needs. They may even tell you that her (your daughter's) needs are not important in this meeting. Ask them why her safety isn't as important as keeping him at home. What they will do when you cannot keep her safe from him? Take her away from you? Gee, wouldn't that add trauma onto terror and injury? I have had them threaten that - to take my daughter away if my son hurt her again while they were trying to tell me it was cruel to have him in a psychiatric hospital where he could get help and we could heal. So they didn't want him in the psychiatric hospital where he already was and they wanted to place her in foster care instead. Be aware that they may offer to take your daughter instead. It will be FAR less expensive and difficult to place her instead of him. It would also traumatize her hugely and she NEEDS you and her home.
Follow your instincts. Take an advocate and/or a lawyer with you to protect YOUR interests and help document what was said. Record the meeting if at all possible - even if you don't tell them youa re doing it. In some (many) states it is legal to record a conversation as long as 1 party is aware of the recording. That means that you can record it if you are part of the discussion and know that you are recording it. You can search online to find out the laws in your state. If nothing else, take a friend or relative with you to take notes of the meeting so that you have a witness to all that they say/do/promise.
Many hugs and lots of strength and support.