Kiara is the daughter of Simba Simba is the son of Mufasa. Kovu is the son of Scar. Mufasa and Scar are brothers. Then that would mean... Spoiler
In the natural world, this is probably normal for lions. But in the Disney sense, I don't think Kovu is actually related to Mufasa/Simba/Kiara/etc. (at least in the end result). He was supposedly adopted by Scar sometime during the first movie (even though Kovu looks pretty much like him). Though, it makes it hard to sift through something like this when there is like a 20 to 1 ratio of female to male lions. (A lot of people always point out the fact that Nala's father is never shown, so naturally...) I always enjoy looking back at old Disney movies and seeing the things I didn't understand as a child. Basically, Kovu is the son of Zira and some other male lion...
This, I think. If I remember correctly, Kovu (who I named my first puppy after<3) was adopted by Scar as an heir.
Actually, the resemblance to scar is way too uncanny. It is very likely that it was just hand-waved, even though the obvious would be to think that he is Scar's son, and thus Kiara's cousin. But as stated above. That's probably normal in the lion world.
Mufasa-brothers- Scar Simba-cousins- Kovu Kiara They're second cousins. I think that's technically legal.
NEVAHHHHH! Kovu was just an heir, said beautifully by Nooka: "Kovu, Kovu, Kovu, Scar wasn't even his father, he just took him in." Yes, I am sad enough to remember lines and I don't care. Although, it would be slightly weird if they did that O.o But then it also begs the question, who was Nala's father? In the world of Lions, it's the King of the pride that mates with all the females... so that would make Simba and Nala.... ;-;
I thought Kovu was just a lion that was forced into being put in Scar's position by that psychotic Zira or whatever her name was. I don't remember too well. It was my favorite Lion King though. I would watch it so many times during a single week.
As Enzy pointed out, Scar wasn't Kovu's biological father, but he did take him in. Zira tried to raise Kovu into being the next Scar, having hatred and such towards Simba and the rest of Pride Rock. That's why he compares himself to Scar throughout the movie. But back to the point, Kiara and Kovu aren't related. Nala and Simba...yeah they probably are siblings. That would be usual behavior for a lion though, what isn't is that Simba only has one mate.
Well, the problem is that these are personified animals. So they give them human qualities, and since the movie is for children, they really don't need to go on specifics regarding Lion behavior, ect. So they just omit the possibilities of having multiple mates, among other things. Lol.