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

Her name isn't menioned in the movie, oder 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 cupcake 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 oder what one) BUT I know her name is Emma in the Bücher ^^

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 Bücher 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. "