I want to clarify that the consistent safe steady person that helps a kid get through could be/often was a family member (or non-familial volunteer) who meets with the child as little as once a week - the key thing is that it is one consistent person who follows the child over the years. It doesn't need to be one consistent person that is doing it all on their own 24-7. It can be as little as an hour or so a week. Really. That is what surprised everyone. The key factor was the long haul.
My hope by saying this is that you can take some of the pressure off yourself to be the main caregiver in his life 24-7 at all costs to your own health.
Even if the worst happened, and he did go back to your mother, it is possible to get court ordered visitation with him as a sister and former guardian. This may be high stress on you, as it may require some kind of interaction with your mother, but I just wanted to mention that there a lot of ways you can be that steady safe person to help him get through this even if you can't be there for him all the time as you would like to be. Or you can be the person that helps connect him with another mentor type figure that can follow him through the years.
My goal is saying all of this is to try and encourage you to have hope and take as much pressure off you that you can. You are doing a lot that many people wouldn't do.
It is EXCELLENT that you are going to go see the family law attorney. They will be the best to advise you and tell you the chances that he could go back to your mother. I couldn't begin to speculate on what the chances are. I am hoping they are slim, but I have seen it happen. It could be a possibility, especially if your mother is doing all that the court required. (I myself was a kid that got sent back to my abusive parents too easily too - I really understand what a disaster it can be for a kid.) But don't fret about this possibility. Team up with a good family law attorney, even if all you do right now is have a connection to one that you can contact if the need arises later on.
Team up with his therapist too, in whatever ways you can. You won't always agree - that's typical. There will always be bumps in the road. But try to reach out to her and connect with her in whatever ways you can. The therapist will be one of your best allies to help prevent him from going back to your mother. They can be one of your biggest allies with CPS and social services too.
If your bother doesn't have one already, you could also look into getting a CASA advocate for him if this ever did go to back court.
In the state that I am in (which is not a big state) there are several residential treatment centers. They are all located in the biggest city in the state and they take kids from all over the state. Here, most kids live there full time 12-18 months.
Talk to his therapist about residential treatment options. You will need her refferal to get him into one. If she is already seeing him three times a week and concerned he is not doing well, she may be really on board with the idea. Be sure that any place you look into is trauma focused, and has a heavy emphasis on treating kids who have been through trauma. These centers tend to be the most likely to help over the long haul, even for kids without trauma histories.
Most importantly, talk to your own therapist too about your own needs in getting through this time of coming up with a good plan on how to proceed and the next steps ahead. This is HIGH STRESS stuff. People without PTSD find it very difficult to navigate all these systems and situations. The more support you can have around you, the better off you and your brother will be for the long haul, no matter what happens. The system is F*CKED up and drives even the most resourced people crazy.
Be kind to yourself as you can be. You are really doing an amazing job with this.