We already know Kiara is in the movie. This means, at some point, Disney is going to remake The Lion King 2: Simba’s Pride. And in order to remake that movie, Disney has to tie up some loose ends. Which means they have to change the story.

So, in order to save themselves from a more, generally, toxic internet fandom culture, to get their “happily ever after” with Kiara and Kovu, Mufasa and Scar cannot be related. If they’re not related, then Simba and Scar aren’t related, which means Kiara and Kovu aren’t related: ergo, no incest.