Hmm, I clicked on this because of the title. I have a teenager who is turning 18 and treats me with no respect. I tried the contract thing. It didn't work. I set rules, it didn't work. He's so big, he forces his way into anything he wants at home, including the wi-fi box, which I had him obey rules about for maybe 2 months. Then, as I need it too, the challenge was how to keep it hidden when he wanted it overnight.

I don't know of any answer for you, except that, I think you said he is 26 or 24, I think he should be responsible enough, from what it sounds like he's been through, to find a place of his own to live.
He's had caring from you, and your sister. Let him find a place of his own, or go to some sort of assistance like welfare, I don't know what you have where you live, and they will get him a shelter or room. Best wishes to you.