I mean obviously they are sisters because Dukat and was in a de facto relationship with Kira's mother long enough to be regarded legally as Kira's stepfather, but the dates are pretty damn squishy here that Kira's mommy supposedly died again just three years before the ravanok was lost with Ziyal and her mum aboard... If you take into account the time it takes for Ziyal to actually "grow up" before she got lost... Well it seems like either their was a name change and a new identity, ie that it was the same woman, or that Dukat was polygamous. I mean he was already bigamous, so what's so bad about being polygamous?