In a telephone interview with South Korea's Yonhap news agency, Ri Kang said the couple had been deeply concerned what might become of them in any power struggle that followed the eventual death of Kim's father Kim Jong-Il.
Ri is married to Ko Yong-Suk, the younger sister of Kim Jong-Un's mother.
"After spending nearly two decades near Kim Jong-Il, I had felt the cruelty of power," Ri said Wednesday. "I thought it wasn't such a good idea to live near that."
Ri and Koh had been tasked with looking after Kim Jong-Un while he was studying in Switzerland as a teenager, and rather than return to North Korea they chose to defect to the United States in 1998.