I was thinking... Zira can't really be Scars mate. There was a deleted scene from the movie in which Zazu tells Scar he needs a devoted Queen, and Scar tries to come onto Nala to bear him cubs, which isn't long before he gets killed.
This was only deleted because it was so creepy according to the director, but that's not to say that it didn't happen, and it was in the stage production of the Lion King anyways.
Unless, they were mates a long time ago, because Nuka is older, and is assumed to be Scars son... maybe she still holds feelings for Scar? But then Scar would have mentioned that he had a son in the first film.
In all honestly, I think Disney really screwed it up and don't want to go back and fix the plotholes LOL.