Since the Z family is so adept at changing appearances and voices, I doubt either criterion is helpful in determining their relatedness. All we know for sure is that their original figures all have the same upper arms and turn blue in sunlight.

Zartan's voice on the cartoon sounds slightly "flanged" (the effect used for Transformers voices); at the least it's re-recorded to give a stereo effect. The mysterious, dramatic quality of his voice is probably chosen and not his "natural" voice. Indeed, his original voice is probably as suppressed as his personality.

Zandar's voice is Peter Cullen's "other" cool voice (after his Optimus/Airborne voice). I think it has a slightly British flavor to it, or at least as British as Princess Leia was trying to sound in the first Star Wars movie. In other words, Zandar's voice reflects a pretention toward sophistication, rather than the real possession of it. I don't think Zandar's using his real voice, either?

So what of Zarana? In the cartoons we hear several of her voices (all performed by Lisa Raggio), and the one she uses when out of disguise is by far the most grating. At least a couple episodes suggest that Zarana is the most redeemable and least rotten of the entire Dreadnok group. Perhaps she chooses the "lower Dreadnok" accent to support her rough, edgy facade.