Star, I really appreciate all that you say here. As for my sister, here's the current sitch: she set him up in an apartment in a city 80 miles from home, pays all his bills, and pretends he's in college at a community college there. He was, but then he dropped out (presumably having scarcely attended classes, from what I've heard), and she keeps paying his bills, and I guess that's kind of a solution, in that it keeps him out of her house, but it's certainly not teaching him anything but that she will cover all his expenses, including spending money, regardless of his conduct (which, as far as I know from YouTube clips and the like, is unchanged) and that is a prescription for a lifetime of paying his way unless she can summon the nerve to cut him off. But I would imagine that the unspoken understanding between them is that he'll stay away as long she pays all of his bills, but will come home and raise hell, as of old, if she ever cuts him off. So she's essentially being held financially hostage by him.
As for my amateur diagnosis of his sociopathy, we can amicably disagree on this, but I'm understandably in a far better position to know than you are. I read EVERYTHING I could find on the topic--Cleckley, Hare, online articles, etc--in '10, including every article on the topic in Psychiatric Times, and he seemed to fit *every* diagnostic indicator in great, unambiguous abundance. I took no delight in arriving at this conclusion about him, but it was helpful in practical terms, as it clarified and simplified my responses to him--i.e., zero interest in his ongoing excuses, manipulations, lies, emotional exploitation and transparently bogus appeals to sympathy, and so on. My orientation became entirely one of protecting family members and property from him, with little interest in what he had to say or complain about it. And that was useful--it eliminated distractions. There are times when "calling a spade a spade" is very useful in practical terms, however unsympathetic it might seem to others.
But I will add, very sincerely, that I don't pretend that everyone else's difficult child situation is similar to the one that I contended with. I have learned that in this forum. Many contexts and many valid responses, of which mine was only one--but it was effective, at least within that limited context. YMMV, and that's cool with me.