Ben was the son of Bob (Jedi) and Xiara after they got married. Shortly after his 5th birthday, his mother was eaten by the Dagobah Lizard, forcing Bob to remarry. Living with his father's new wife (who was not his mother), Ben was given a half-brother, Yod.
As Yod grew up, he argued with his older half-brother, who in turn found out a way to stop him from annoying him. Ben recalled to his half-brother the fact that they have the same father, but not the same mother, and then said that they were not brothers, but instead half-brothers. This made Yod nearly cry.
21 years later, Bob was killed by Count Dooku, when Dooku turned to the dark side. Ben and Yod sought to avenge their father and thus became apprentices of Yoda, who trained them well and told them to be patient.
In their first battle with Dooku, both brothers were severely wounded, but they were then astonished by the surprising appearance of Yoda, himself, who wisely fought off his former apprentice and defeated him.