This is a volatile situation that needs to be addressed yesterday. Not only is it unhealthy for you but think about your sister who actually has to live with this control freak.

Two things I'd like to address:

Frankly, though I feel for my sister, she is an adult. I can't save her from the imprisonment she has grown accustom to.

And I am totally powerless to do anything for my sister.

Wrong. You CAN save her from this creep. There is strength in numbers. You and your sister have every right to confront this bully if your sister won't do it on her own. It sounds like he's out of control and the more he gets his way, the more empowered he will feel. I'm afraid it will get worse as time passes. I would think your sister is miserable in this marriage (even if you say she loves him). While I don't advocate for divorce, that might be an option. And counseling? Good Luck. This guy sounds too hard-headed to admit fault so I don't think it would do any good (although I could be completely wrong).

All my best to your family. Life is too short to put up with abuse, no matter the form.