I wouldn't base much on what wookiepedia says (or any wiki really.) So far as I can tell from the sources they cite, the whole thing is sort of skirted around. Ventress and the Nightsisters are "Dathomirian" while the Nightbrothers are Zabraks. No mention is made on whether there's any cross-breeding or not (though, if not: where are the Zabrak women and the Dathomiri men?)
It's all rather vague and glossed over so far as I can tell.
Given that we've seen female Zabrak elsewhere and they don't look like nightsisters, I'm inclined to think that there's another colony Zabrak that's all female and the Nightsisters keep them separated from the men to control and manage their bloodlines.
As for why there are no Dathomirian males: I would submit that perhaps there simply aren't any anymore and that the Nightsisters use there freaky "magiks" to make new baby Nightsisters.
