Rachel:
You can't be your brother's keeper.
He is an adult.
He is active duty military. (sorry - just saw your edit) he needs to enlist full time in order to get the benefits of the help he needs and a stable living arrangement.
He cannot control HER. PERIOD. She will do what she is going to do. Whether or not he is in her life or not. If she is suicidal - then it's on HER NOT HIM. I know that sounds cold and unkind - but it is a fact of life - NO ONE can control another - she will do what she is going to do whether or not he's there or not. I cannot stress to you enough this is a FACT.
If your brother enlists full time - he can live on base in the barracks - it will save him money and he can live on his own - no room mate.
If he goes full time military - he gets paid on the 1st and 15th of every month. If he has mental problems - the military WILL get him the help he needs in order to become a functioning member of society and the military.
If he doesn't like school - you can't change that. You can lead by example and show him the benefits of a great education - and military can do that as well - ONLY HE CAN IMPROVE HIMSELF.. YOU CANNOT DO IT FOR HIM. And he has to WANT to improve himself.
Even though it is hard - you cannot let your brother move in with you and your family - you will only enable him and let him be coddled further. He needs to man-up and go to his 1st Shirt or his Commander and spill the beans - they WILL get him the help he needs to break this relationship, improve himself and get better.
He is on the path to doing the right thing - he is finally admitting he is in a bad relationship and wants out. YOU CANNOT DO IT FOR HIM.
This is like an alcoholic or addict - the person has to WANT the change and BELIEVE they can make the change for himself. YOU can want it all you want. And you can jump through hoops and spend tons of money on him - but none of it will work unless HE wants it.
You MUST let him handle this on his on. He MUST understand that she will do what she is going to do and NONE of HER ACTIONS ARE HIS RESPONSIBILITY. PERIOD.
Have him go to a Priest, Chaplain, Rabbi - whatever religion and let him help himself - you can give him the tools - direction - but DO NOT give him money, do NOT give him a room - he needs to get out of this on his own.
My prayers are with you!!
Cheryl