Cast question: Name (as alternative name)

I was looking over the cast of STARDUST (2007) and saw this entry:

This piqued my curiosity, so I went back to the DVD and checked the credits list. It had this:

    Yvaine's Sister  .......   Coco Sumner.

Why had nobody ever attached the alternative name? So I submitted the alternative name, only to receive an odd response: something like "This information cannot be changed." Curiouser and curiouser! The cast member in question identifies on IMDb as an actor, yet in this film is taking a small female (non-speaking) role. His birth name is given as 

To add drama to the mix, his father is the musician Sting, of the Police. (Both his children appear in this film in small roles.)

IMDb has famously refused to change or withhold actor information which the actor deems embarrassing to themselves (for example, correct birthdates), in the interests of accuracy and completism. Are they making an exception in this case? What is the story, exactly?

                                                                hutch48