Very sorry to hear this OTWO. You are a great brother to be helping your sister so much.
My sister was in a very similar situation with her POS husband as she was dying last year. Her situation closely parallels your sisters. BIL refusing to work, asking MIL to pay the bills etc. Sis got a restraining order against him a few years ago. Then when she knew she was terminal she allowed him to be power-of-attorney. That sealed her fate. At least, right now, your Sister is letting you help her. That's good.
Hang in there. Try to remember any positive memories of your sister. You'll cherish those later.