Remeber the scene where Nuka is complaining about Kovu. For a few brief seconds his mane is full, then becomes a young adults mane.
This ( I won't call it mistake at risk of being hounded over again) would further suggest her was alive during Scar's reign and was Scar's son.
Also as a teen Nuka has freckles(that appear and dissapear simlar to Kiara's inner leg coloring) but once he becomes an adult they dissapear. This could be proof since Vitani and Nuka both have/had freckles they are full siblings while Kovu is the only half sibling.