Rex Blackadder was a Navy sailor who was the brother of FBI-turned-NCIS Special Agent Vivian Blackadder.
Tragically, Rex died on the attack on the U.S.S. Cole which occurred in October 2000.
Not much was known about Rex Blackadder but it's implied that he had a good relationship with his younger or older sister, Viv.
JAG Season 8 Edit
Rex's murder in the attack on the Cole left Viv greatly devastated and consumed with grief.
It also resulted Viv in becoming hellbent on avenging Rex's death by any means necessary which also saw her quitting the FBI and joining NCIS nearly two years later in 2003 and presumably a few months prior to "Ice Queen" all for the sake of becoming a member of the kill team that would target Hussan Mohammed, the man who had arranged the attack.
Unfortunately, Viv's dreams of avenging her dead brother were shattered upon her actions during the Attempted Terrorist Attack In Rota, Spain which forced the anti-terrorism teams to act quickly and even though they managed to stop the attack with Gibbs even killing Mohammed, Viv was later presumably either sent back to the FBI or forced to resign from NCIS for good in disgrace for jeopardizing such an important operation.