I know this sounds far-fetched, but it's possible. We don't know Zira's age, except that she's presumably an adult by the time Kovu, who is NOT a child of Scar, is born. Kovu may have been the child of some lioness and a rogue lion. Perhaps the mother abandoned him and Zira, perhaps a teen or young adult, adopted Kovu. We don't know how long past his coming of age Simba was in exile. Perhaps if Zira was born a few months after Scar took over, she'd be old enough to be a parent to Nuka sometime before Simba's return. Vitani perhaps could have been born AFTER the exile (we don't know her age, though she looks a bit similar to Kovu's age or younger.) Perhaps seeing that his daughter's firstborn, Nuka, was not fit to be the new heir, and wanting a fit king, picked Kovu to be the successor.
Zira may have been quite normal until her father's death, at which she became insanely obsessed with avenging Scar's death by killing Simba and taking back the throne. Since she was Scar's daughter, rather than some random Outlander with an unknown relationship to Scar, she may well have been able to persuade some of the Pridelands lionesses to join her side.
Of course, I don't know why she would never have referred to Scar as "father" at all, so this theory seems unlikely. But is it even plausible?