Thanx

I always had problems in telling if Zira and Scar are Vitani's parents. I think that Nuka definitely is their kid but Vitani is about Kovu's age that makes it less possible for her to be their daughter. Tama made me even more confused because she really does look related to Vitani.
Anyway, Tama appears in comic book, not a movie storyline and there is no official answer if she's related with any TLK character.

I doubt Zira stole her, she could inherit look from other parts of family. I already saw situations where son or daughter really is not looking like one parent, but that parent is real. But fans made theories as well as they made ones saying that none of Ziras cubs is Scars. Also Vitani refers to Zira as mother and also the way she acts towards Zira, leads me to believe that they are related. I say that Nuka, Vitani and Scar are not related to Scar with more certainty that that they are not related to Zira. Also Ziras personality might be changed due to loss of Scar, Simbas "betrayal" and exile. All hitting you at once can leave scars on person. Certain stuff just change people and zira could not let anger and revenge to go away. While Vitani might reflect Ziras personality from before Simbas return.

But that is my personal opinion and arguments can be found to confirm your side. So I can only say that it is up to you what to think of it. TLK universe is very open to interpretations on certain matters. And I think you made another good theory.