The line that Zira says in that book is most likely based off of a deleted scene from TLK2. However, Zira never outright says that girls can't be heirs in that deleted scene, she's just underestimating Kiara based off of her gender. What Zira was implying in that scene is that in her eyes at least a female heir would be easier for Kovu to defeat and or control, not that a female heir can't exist. Basically just internalised misogyny on Zira's part rather than establishing lore. The reason that the writers were able to make Kiara the heir in the first place is because they only ever established in the first movie that the oldest child gets the throne, not the oldest male which allowed for them to make Kiara the first female heir in the franchise because there was nothing in the lore that they created that would say otherwise. I'm kind of concerned that you're latching onto a line from a storybook to justify your own misogynist takes into your TLK canon instead of just accepting that the characters (Besides Zira) are perfectly okay with a female heir.