Kiara and Vitani are lovers, Kiara simply used Kovu because she thought that by marrying him, she could create peace between the warring prides and allow Vitani to live in the Pride Lands.
Zira is Scar's younger half-sister (Scar being Mufasa's younger half-brother) and she actually manipulated Scar initially, driving him to his evil tendencies because of her darkness and insanity, which was partially brought about by their mother neglecting Zira on account of her being a rape baby. (I've talked more about this theory elsewhere).
When discussing what is actually canon, I don't bring up either of these things as I fully admit they are just weird ideas of mine.