I know that there have been others who have writen an article about this but I have yet to find one that actually makes sense and more importantly, one that contains accurate information.

So to start off, I will explain the situation at hand.  The most popular belief for Zira's cubs is that they're all her biological children just with different fathers.  There is debate whether Nuka is Scar's blood son because of his age and appearance, but because he doesn't encourage this as one of his main arguments for becoming king, this theory has been heavily questioned (see Mahewa's article Nuka's Father for more information).  It is said that Vitani is Scar's daughter but agewise, she doesn't appear to be.  Another theory is that she is Kovu's litter mate/twin sister, but Zira specifically says that Kovu "was the last born", resulting in once again, confusion among fans (unless Vitani is the older twin).  Lastly, the animators clearly say that Kovu is not related to Scar whatsoever, however he was apparently born during Scar's reign as it states in the movie that Kovu was adopted and hand chosen by Scar to become the heir to the throne, but this couldn't be possible as this would make him an adult by the time Kiara was born. 

Most of these mistakes were resulted in the animators inaccurate timeline, the rest however, still remains shrouded in mystery. 

Now moving onto my opinion.
 

I support those who think that Kovu and Vitani are twins, but I disagree with the fact that they are Zira's biological children.  Most people might respond to this with "wikipedia clearly states that they are her children", or "in the song My Lullaby, she calls Kovu 'my son' and 'my little kovu'".  Dear readers, wikipedia is not the most accurate site out there, AND if Scar really did adopted Kovu like everyone says he did then of course Zira will call him "son".  Anyways, Vitani on the other hand, bares a strong resemblance to Zira as an adult so she MUST be her daughter right? Wrong.  If you haven't noticed, most of the lioness' in Zira's pride kind of look the same, so I think Vitani was the daughter of another outlander lion who possibly, abandoned the two cubs or, died of some unknown circumstance and left her children in the care of Zira, afterall, the hyenas pretty much destroyed the Pridelands under Scar's rule so it's possible that their mother couldv'e died of starvation.  By the way, I think I should also point out that at the very end of the movie, Vitani is the first one to realize that they are "one" and crosses over to Simba's side, Zira says "if you will not fight, then you will die as well.".  Why would Zira say that to her own daughter? ESPECIALLY if she were Scar's cub too. But even so, she did prove to be able to show affection for her cubs in a few rare occasions (e.g. Nuka's death), so I find it out of character that Zira would've threated her own "daughter" with her life.  Now, Nuka, is a lot harder to argue about, he doesn't ever make a referrance to Scar being his father, but he looks just like him and he appears like he could've been born in Scar's reign, that's what seems to be throwing everyone off.  However, Nuka appears have drank out of the Fountain of Youth since he never ages throughout the movie, so he either looks a lot younger that he really is or he's immortal, (that's the best I can come up with haha).  I really don't know what to think of him yet, just that it's a strong possibility that he's Zira and Scar's biological son, perhaps Scar didn't give him much attention and therefore Zira thinks that Nuka isn't worth her time either, it would make sense that Scar didn't choose him as the next future king, I mean look at him... he's kind of.. well... how to explain it... stupid.

And that concludes my first article, thanks for hanging in there, I know it was long but as you can see, I had a lot to say.