Most likely a meant for the game but was not used in the final finished game.Thats why she can only be seen using the SP.
I know! If she died and you wanted to do the story again, the hidden Nanihi replaces her if the original one dies/grows up. (sorry if I'm wrong I only have sims 3 this is just a guess)
I honestly think all the hidden duplicates of Fangaloka, Huhana, and Nanihi are there because they were prototypes of the actual character and EA simply removed their data once they finished the real ones. That, or they are possibly ghosts of the real ones.