Twicrack Addict has the scoop: (Warning possible *SPOILERS*)
TwiCrackers, meet Christie Burke who's been cast as teenage Renesemee in Breaking Dawn!
Christie - aka second luckiest gal in the world (first place still goes to Kristen, natch) - apparently gets to make out with Taylor Lautner in a flash-forward to Jacob and Renesemee living happily ever after...
Whether this is a Bella flash forward or when Jacob first imprints on her (as previous rumoued Lainey script deets detailed a Jacob/Renesemee montage) will remain to be seen.
According to her representation, Performers Management Christie is 5'7, has hazel eyes, brown hair (though her pic totes look more auburn) and weighs 115lbs (plenty light enough for Jacob to spin her around in throws of adoration, etc).