Relativity in Ender's Game

[splashpot]

Pretty basic relativity questions. Have any of you read Ender's Game by Orson Scott Card? Great book by the way. But I'm questioning if the issues with relativity in the book are accurate.

In the book, a military legend, Mazer Rackham, defeated an army of alien bugs 80 years ago. Realizing that his expertise would be needed in the future when starships are ready to attack the alien homebase, they decide to put Mazer in a spaceship, send him off at near light speed, and then have him return, just in time to train the next military genius, Ender Wiggins. Mazer says only eight years passed for him on the ship, but 80 years passed for everyone else. This does not sound right to me because on the return trip, any time that was gained for Mazer would have been lost. Am I wrong?

Second. In the book, they have a device that is capable of instantanous communication across the universe. While I don't expect that to be possible because it is way faster than the speed of light, they use this device which makes me wonder about another age question.

At the end of the book, Ender decides to leave on a ship with his sister. He leaves behind his brother on Earth. For Ender, only 2 years passes on the ship. But for his brother, 50 years passes. Ender uses the communication device to talk with his brother who is 77 years old, while Ender is still young. Is this possible? This seems wrong to me because Ender flew away from Earth, which could also be viewed as the Earth flying away from Ender. It doesn't make sense to me that Ender should stay young while his brother gets old.

I know the book is fiction and that an instantaneous communication device is probably impossible, but are these other relativity concepts flaws in his book?