I think you're screwed. If he's a deadbeat - he's a deadbeat. Nothing you can do about it except learn to deal with it. You just have to differentiate between your sister and him. Be nice to her, be useless with him.

As for pounding him; you can't lay a hand on him as long as he doesn't lay a hand on anyone in your family first. You might win the battle but you'd lose the war.