"He has made the world weep," the 2004 Princeton University graduate, who now works as a contractor for the US state department office, said on behalf of the grieving family.
"Our family is so very sorry for my brother's unspeakable actions. It is a terrible tragedy for all of us."
"We pray for their families and loved ones who are experiencing so much excruciating grief. And we pray for those who were injured and for those whose lives are changed forever because of what they witnessed and experienced," she added.
"Each of these people had so much love, talent and gifts to offer, and their lives were cut short by a horrible and senseless act."
Ms Sun-Kyung also expressed the family's shock at her brother's actions, saying: "This is someone that I grew up with and loved. Now I feel like I didn't know this person."
"My brother was quiet and reserved, yet struggled to fit in. We never could have envisioned that he was capable of so much violence."
The incident is the deadliest school shootings in US history, in a country where gun crime and gun ownership are extremely politicised issues.
A memorial service was held at the university's sprawling campus on Friday which also marked the eighth anniversary of the Columbine high school massacre which saw 12 students and a teacher shot dead.
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