Moving in with Mom

Well, as I said, my brother is in rehab, again and we can only hope this time it works..
He's been there for a month already and has 2 months to go, Hopefully he will make it.
For the first time, he is making arrangements for assisted living, but he has gone six months before, before falling off the wagon. my only hope is that he get's accepted so that he doesn't run back to mommy. He's 47.
The thing is, I, like so many other people am going through some rough times, finacially and with work.
I left home at 18, and i am now 48, and in the last 30 years i have never had to move home and my mother and father have never had to bail me out financially or otherwise.
They have always had their hands full with my brother.
So, now Im having some troubles and up until now, my mother has not offered to help me or to even invite me to stay with her. The extra room always needs to be available for when he comes home.
unfortunatly, as I said before, my mother is the enabler, and there is no changing her. She doesn't understand that my brother has to learn to stand on his own 2 feet.
So, last week out of the blue she asked me if I would like to move in, at first she wanted me to pay $500.00, but when I said no thanks, (paying or not, i can't trust that she won't let my brother home if he screws up). two days after that she called to apologize and offered for me to live there for free, so I can save up some money...She promised that under no circumstances was he coming home. He and I can not live under the same roof, that's for another blog, so now Im deciding what to do.
tomorrow I go to visit my brother and see what rehab looks like, he seems to be doing really well, and Im very happy, BUT, I have to let him know that I plan to be the buffer so that no matter what happens he doesn't come home. My mother is a mother and she doesn't keep him a drug addict on purpose, but babying him just sucks him back to square one.
When we spoke the other day he told me he didn't want to go home, i think he realizes that going back is poison for him. so instead of being a bitch, I going to try the good sister by living with my mom so there is no room for him.
it's time he grew up and gets the chance to start again on his own.