It does make a bit of sense. The pointy features and beard. The light brown fur and red eyes. It's a very popular theory and fills a lot of plot-holes such as, Nuka's origins, Outsider's origins, Outsider's banishment.
It would also be awkward for Sarabi to give little Zira baths since she thought her Simba was dead..
But if Scar and Sarabi were Zira's parents. Why did she became the murderous lady she is in SP? Maybe Scar manipulated her to become like him. Promised her that she would be a Queen and when Simba came back, he 'killed' her dad and I think Zira LOVED her dad. But what if Scar never told her that she's his daughter and Nuka is the result of their inbreeding?! But lets make it ten times WORSE!
What if the Outsiders were born as a result of Nuka and Zira!? (Which would explain Nuka's insanity) and Scar and Nala!? (By this I mean that Scar and Nala resulted in the other Outsider lionesses we see in SP.. and they also have Pridelander noses.. and also resulted in Vitani.)
Also you have to consider the fact that Incest is a natural thing among Lion-Prides and the genetics actually make it possible. It depends if you believe Scar and Mufasa's mother Uru had Green or Red eyes and if Sarabi's parents had Red eyes. (Yeah I did a lot of research XD..) but since there has to be two carriers of the gene, then Sarabi most likely carries the 'red eye' gene and is able to pass it on to her children. E.g: Simba. And I get the weirdest ideas for TLK
What do you think?
I know it's very DISTURBING.. *Gives vomit bags* but hey. It makes for an interesting theory AND I wanted to THINK really outside the box.. and I just had a brain-wave!