Gregg, who frequently visited her father's home in Cleveland, Ohio, and ate with him there hours before his captives emerged, said she hoped her family would not be tarred by his alleged crimes.
"They definitely are not a reflection of myself or my children," she told CNN. "We don't have monster in our blood."
DNA tests have proven Castro fathered Jocelyn, who was born to Amanda Berry, one of his victims, on Christmas Day in 2006.
Gregg said Castro had shown her a picture of Jocelyn, describing her as "my girlfriend's child" before dismissing Gregg's suggestion that the girl could be his daughter.
Insisting she and her siblings had no idea of their father's actions, Gregg conceded she was now making sense of his odd behaviour and abrupt disappearances: "It's all adding up."
Handwritten notes from Castro's house are said to state "I have a problem with my head" and blame childhood abuse by a relation.
His daughter Arlene, who was the best friend of his captive Georgina DeJesus at the time of her 2004 kidnapping, said she hoped the two could be reunited.
"I am absolutely so, so sorry," Castro, 22, said on ABC News. "I really want to see you, Gina, and I want you to meet my kids. I'm so sorry for everything."
Castro is charged with kidnapping and raping Berry, DeJesus and Michelle Knight, as well as kidnapping Jocelyn. All four escaped from his house on Tuesday.
