Theres always a lot of debate over whether or not Scar's cubs are his or not. Nuka and Vitani are usually accepted as his cubs, and Kovu widely accepted as not being related to Scar. It's only been the recent years that Vitani has really been debated about.

And so here I am to be the odd-ball to the fandom and present ideas as to why none of them are related to Scar. However, keep in mind, that I am a fan of Nuka being Scar's son, before people think I'm hating xD

First, let's analyse some dialouge, shall we?

Let's start with the very introduction to Nuka and Vitani.

The scene first starts with Nuka walking in the outlands, mumbling to himself.

"Kovu,Kovu,Kovu....Scar wasn't even his father...he just took him in.."

He then talks to Vitani, and after arguing a little, Nuka continues to say:

"Ah! Who cares?! I should have been the chosen one! I'm the oldest! I'm the strongest! I am the smartest-!"

If Nuka was Scar's son, wouldn't he have mentioned so? Wouldn't it have been the very first thing he would have mentioned, in his argument as to why he should be king? No. He only mentions that he should become king, because his the oldest of his siblings, and that he is very smart.

He refers to this also in a deleted scene:

  http://www.youtube.com/watch?v=leiPvxuNop4 

Pay attention to what Nuka is saying.

"Hey...speaking of kings. I was thinking...that since I'm the...oldest...maybe I should be king! huh!"

Again, he doesn't mention a word about being Scar's son. He had a clear opportunity to suggest that he could be king instead of Kovu, and surely, he would have used the best reason as to why he should? Shouldn't he have said: "Hey...speaking of kings. I was thinking...that since I am Scar's son..maybe I should be king! huh!" 

And now, let's address another reason.

Throughout the film, Vitan and Nuka always refer to Scar, as just 'Scar'.  Surely if Scar was their father, wouldn't they call him 'Father' or 'Dad'? Wouldn't have Zira taught them that Scar was their father, and he should be addressed as so?  Just a thought.

And now, here is a (deleted) scene that was only released recently, that might also help back-up my point:

http://www.youtube.com/watch?v=I0WATOVmLtQ 

Scar decides that he needs a queen. And his first choice is Nala. Nala, in this scene, is an adult, and there is no Zira in sight. After Nala rejects Scar, she runs away. Now, It took Simba a few days to reach the Oasis where he would live for the next few years. Nala would surely reach the same place in a few days as well? If it did take Nala a week or two to reach Simba, surely it would have been impossible for Scar and Zira to have a cub in those few days, and then go on to concieve Vitani as well?

Perhaps Zira had already given birth to Nuka, when Scar chose her as queen.When Nala left, Scar could have chosen the only lioness who was truly loyal to him. Perhaps Nuka was from a different lion, which is why Scar rejected him as heir. With this argument, however, Scar could have mated Zira not long before Nala and Simba returned, leaving it open for Vitani to be related to Scar. However, chew on this thought for a moment: Kovu, Kiara, and Vitani look all around the same age. Perhaps Kovu and Vitani are from the same litter. Throughout the film, it is stated over and over that Kovu is not Scar's son.  If Vitani and Kovu are from the same litter, then how would Vitani be related to Scar, if Kovu wasn't?

Some may argue, that if a lioness mated with two different lions while she is still in heat, she could produce tWo cubs two different fathers. But by this point, we know Zira is completely devoted Scar. Why would she run off and mate with another lion, when she had Scar as a mate?

And those are my arguments against Nuka and Vitani being releated to Scar ^^ I applogize for the massive amounts of text.

If you have any questions, I'll be happy to answer :3