The Wu-San sisters were the granddaughters of the O-Sensei. Their mother, who had been raised in Japan, had fallen for a local gangster and when she became pregnant for the first time ran off with him to Vietnam. When the gangster found the girl to be pregnant a second time he sold her and their first child Sandra to the League of Assassins. The woman and child were taken to a training camp deep in the bowels of Vietnam. Carolyn was born soon after and the girls were raised and trained to be killers. This was where David Cain would first encounter the sisters.
When Sandra was seven and Carolyn five their mother dropped dead in the fields and was buried in an unmarked grave. Three years later their grandfather the O-Sensei finally found them and, after tearing apart the training ground for the greatest killers in the world single handedly, brought the girls back to China with him. They were then raised just as any student of the O-Sensei's dojo. Sandra would eventually try to establish a life for herself in the outside world while Carolyn stayed to help run the facility.