I made Nala and Mheetu's father a rogue also, actually.
However, I can believe that either Mufasa or Scar fathered one or both of them, since Nala basically got all her looks from her mother anyway and Mheetu never got an 'official' design but looked really pale in concept art.
(However I included some incest on my own tree regardless, I made Sarabi, Sarafina and Zira all sisters)
Though in the animal kingdom, incest isn't exactly frowned upon, so I don't think it's as big of a deal as some people make it out to be with those characters xD