OK, I once heard a theory about Scar and Zira being related. This is why it isn't possible
First of all, I would like to say that I am not pointing at anyone here, because I don't mean anyone in particular. But one theory I heard was that Scar and Zira may be related. I'm sorry, but how does that work? Let me go over it.
"Scar and Zira must be related, because they look like each other!"
If you mean through colours, then not possible
"What? But Mufasa and Scar don't look alike, and they are brothers!"
Exactly. Scar and Mufasa are brothers. That is canon. By saying that Zira is Scar's sister, that also makes her Mufasa's sister. And Zira being Mufasa and Scar's sister is not canon as far as I can see. Lets look at this. If Zira was Scar's sister, and Scar killed Mufasa and chased Simba away, do you really think that Zira would worship him like that?
"Well, Zira might have hated Mufasa and Simba!"
And why would that be? She might hate Mufasa for maybe being mean to Scar in the past, but I doubt it. And also, she might hate Simba because he was the son of Mufasa, but she wouldn't want him to die just because of that. I mean, that's too immature, even for Zira.
Why else could Zira hate Mufasa and Simba? Obviously for the same reason as Scar, wanting to be queen. In that case, wouldn't she be plotting against him, too? He would be in the way of Zira becoming queen. She wouldn't fan over him like that now, would she? So that's one reason why Scar and Zira = Brother and Sister is impossible.
"Scar and Zira have the same body build! That must mean that they are related!"
That isn't a good reason, either. All outlanders have a skinny build. I'm sure that Vitani does, and Nuka. Does that make them the brother and sister of Scar as well? Of course not. Again bringing in Mufasa and Scar. Mufasa does not have a skinny build, and he is Scar's brother. They do not have to look alike to be related. So that does not prove that Zira is Scar's sister, either.
"Well they have the same noses, don't they? They must be related!"
Let's take a closer look at the outlanders. They all have outlander noses. Mufasa, once again, does not have an outlander nose, but he is related to Scar. Technically if you look at it they don't even have the same outlander nose.
"What do you mean? Of course they do!"
No, they don't. And this is why. Please look at the following two images.
Am I the only one who sees a difference? With Scar's nose, the inner part is more, how do I put this, round maybe? And Zira's nose on the inside is much straighter. Clearly getting rid of that theory.
"Well, they have chin tuffs! They have to be related!"
Once again, not possible. Yes, they have chin tuffs, but if you look at the images above, Zira's is different to Scar's. Scar's seems to be going in, towards his mane. Zira's is much smaller, and it goes out. Deleting that theory. In fact, if you also look, thet don't even have the same eyes. I know the eye colour is different, but I didn't mean that/ Look at the eyelids. Look at the upper eye and lower eye. Zira's is pointing more to her ears while Scar's seems to be pointing more to the top of his mane.
Now lets look at a few other things.
Assuming that Nuka is Scar and Zira's cub (a hint that we get from the quote "Kovu Kovu Kovu, Scar wasn't even his father, he just took him in.") if Scar and Zira were siblings, then surely it would be wrong for them to have a cub together? I am well aware that in the wild lions mate with their families all the time but this is a children's film. I'm pretty sure that they are supposed to represent humans more than real lions.
"And I his loving devoted queen." A quote from Zira in a deleted scene. If Scar and Zira were siblings, why would Scar make Zira his queen? He wouldn't, that's the point.
Now then. Scar chose Kovu to be the next king, right? And Zira mothers him, am I correct? Why would Scar ask his sister to raise a child that was the chosen one instead of asking another lioness? Let's think about this. Zira trains Kovu to become the next king, and it looks a lot to me like she wants to try and impress Scar's spirit. She wouldn't do that if she was his sister. She would try and get revenge herself, not make his chosen one do it. However, if she was his mate, or a fangirl, she would snap up the offer without a seond thought, right? She would be happy to follow it the way that Scar wanted her to (sorry, you're probably lost now) because she wanted to do everything his way. I can't see a sister doing that.
I hope that things are a little clearer on this matter now. If you still think that they are siblings, then that's fine. I'm not trying to change anyone's opinion, just show them why it couldn't be and let them decide for themselves. Oh, and try to keep things friendly down there in the comments. I mean, there's no need to get into an argument over something that doesn't really matter. It won't make it the end of the world, right?