Tanya isnt going to be in Eclipse so until they offical give the green light for Breaking Dawn...its anybody's guess....I did find this on the web...
Lopilato & Michael Buble
Twilight Lexicon confirms that a recent Argentinian website incorrectly reported that Luisana Lopilato was cast in Eclipse as Tanya. In fact, Tanya doesn't appear in Eclipse:
It turns out despite the fact that it’s being reported as fact in various quarters that it’s just plain rumor and nothing more. We confirmed with Summit that Luisana Lopilato in not invovled in Eclipse