Just a theory I've been playing with that I felt like sharing. 

What if Zira is the daughter of Scar and Sarabi?

Zira has very similar fur colour to Srabi, for one. They both have darker eye-shades and both have red eyes.

And obviously she has the looks of Scar. The skinny body, nose, the overall pointed look Outlander lions have.

I know some people could argue that Sarabi has orange eyes, but it is still possible for a cub to in-herit red. Mufasa is Scar's brother, and he obviously has red eyes. This means somewhere in Scar's family line someone carries Red eyes, right? The same could be said for Sarabi, especially since we don't see her parents in any TLK media, so it is all up in the air.

As for her Stripes and ear-strips - well, the same could be said for them. Perhaps Uru or another relative had a stripe or ear strips, this would mean Scar carries them in his genes. The same could be said for Sarabi, so I'm not going to repeat myself.  (Plus Sarabi was seen with a stripe in some concept art - just thought I'd throw that in there)

Now I've explained why I'm going to tell you how I think it could work.

We are never told how long the time-gap between when Simba runs-away and when he returns really is, so we can pretty much assume anything happened. All we know is that Simba is a young adult when Nala finds him and brings him back.

This would mean that there is perfectly enough time for Scar and Zira to have a cub, and let it grow to at least a young adult, and possibly even give birth to Nuka and become pregnent with Vitani and or Kovu (depending on whether or not you believe she is his real mother or not)

Perhaps Sarabi was still Queen when Scar became King, and due to Scar pressuring her into having a heir, or  perhaps due to tradition (you decide) they had a cub. Unfortunatley, Sarabi had a female, and Scar saw her as unable to be the heir due to being female. Then, perhaps Scar took Zira away and raised her himself, which would explain her devotion to him and her behavior.

These are my reasons as to why I believe that Zira is related to Sarabi and Scar, I hope you enjoyed reading my article. :)

If you have any questions, feel free to ask :3