Dear Rebel Mommy
My son is in a sober living home through the Rescue Mission in my town. While he is able to pay through his SSI, it is free for indigent people. Included is room and board and program. These kinds of programs are all over the country. Your daughter does not need you to house her or feed her. She can go to a program like this any time she chooses. If she is mentally ill, she can apply for SSI and receive a monthly payment. The reality is that our children prefer to rely on us because they believe they can impose their rules and use us to support their addictions and bad behavior. We're easy.
I think you are very wise to know and to enforce sensible, responsible boundaries. Not only is this necessary for you and for your younger children, this benefits your daughter too. My son only entered the sober living home (twice, 2 years apart) when I said "no." If I look at myself in the mirror, clearly and squarely, by housing and feeding him, I enabled him.
It took me a very long time to have the clarity and the strength to do the right thing. I support you completely. Welcome.