I don't think it's Barriss either, (for previously stated and obvious reasons). Also, the male inquisitor apparently is titled "The Fifth Brother," so I have the feeling the two belong to some other kind of faction, cult, etc. with at least seven members or "children" and these two were the chosen ones to hunt down the rebels.
I do like the idea of them being descendants of Dathomirian witches or something.
BUT, I guess we won't really find out until Lucasfilms & Disney release further information, or even until the episode debut.
(I honestly wasn't planning on following "rebels" but it's starting to sound interesting, especially with Ahsoka).