Brother's Trilogy:

On a planet colonized by Earth more than 1000 years from now, humans have divested themselves of much of the technology used for war. For years there is peace until they discover a number of indigenous races, most hostile. The monarchies conclude that the only way to preserve their way of life while protecting their kingdoms and especially their kings is to establish an elite group of warriors chosen from the firstborn son of each ruling family. The firstborn is taken at birth and forgotten by all but his parents until the day their next eldest son takes the throne. Then the eldest brother comes to serve as guardian of the king and champion of the realm.

The story is of the love brothers can have for one another and their willingness to live and die for one another.