I believe Sinning Hand is simply cbi
Well this is what I used to think, and how I've always seen it played.
However, when I stopped to think about it (ironically when The Guardian mentioned Sinning Hand in the "What EEs are CBI" thread yesterday), it occurred to me that EEs are CBP/I/N when played, and only then.
Sinning Hand has very unique wording (nothing else like it, really), and the "out" that it provides an opponent is not an Instead ability (which is the common way to word abilities like that), but a "Negate" ability. So when Sinning Hand is played and the opponent discards half their hand, Sinning Hand is not CBI.
At that point, if it is targeted for negation, Sinning Hand can't "go back in time" and retroactively gain CBI. It had to gain CBI when it was played, or not at all.