There's been plenty of hints dropping that season three will focus a lot on Jack's backstory and his time in Vietnam. Michael Angarano has been cast as Jack's brother Nicky, and it looks like he'll play a pretty big role in the new season. Jack clearly has a lot of baggage when it comes to his brother's death, so it's possible that the reason it was even more tragic was because Nicky dies in an attempt to save Jack.