Incest??? They're LIONS for Pete's sake! Sexual morality is an alien concept to them… The King of the Pride gets ALL the lionesses as his harem, including his mother and sisters.

And yes, Kovu is Scars adopted son, although judging from his character design he was obviously intended to be his biological son originally.

Additionally, royalty often resorts to marriage between cousins: Queen Elisabeth II married her cousin, so did King George V, King Edward VII and Queen Victoria, and that's just in Britain.

Meh, incest is relative.