How is the Jack's sister known por her name? is her name in the movie, o in the books, o both?

Her name isn't menioned in the movie, o the movie credits, she's just 'Jack's sister'. Though it says 'Jack's sister/Pippa', Pippa is a different character (one of Jamie's friends, the one who hits magdalena with a snowball) and the voice actress voices both characters.
Though I haven't read the books, her name is mentioned in them (though I have no idea where o what one) BUT I know her name is Emma in the libros ^^

Actually... I dont think Emma is the real name of Jack's sister. Nor Pippa...

Her name was not mentioned in the movie nor even the libros i think... According to the Jack Frost - Rise of the Guardians Wiki (file:///D:/My%20Documents/Jack%20Frost%20-%20Rise%20of%20the%20Guardians%20Wiki.htm) , "Her name has not been confirmed as of yet. "