Best wishes for your sister in terms of DTMFA-ing this creep. If she doesn't seem inclined to... well, forwarding her this Onion piece
may not help, but I can't think of too many other remedies offhand.
You haven't shared your wife's take on all this. Regardless, the healthiest thing you
can do is probably just to do your best to support and affirm her position (assuming it's the right one).