Fearful that the man would come back and take her too, Mary Katherine Smart quietly remained in bed for two hours until she got up the courage to tell her parents that her sister had been kidnapped.
"I thought, you know, be quiet, because if he hears you, he might take you, too, and you're the only person who has seen this and you have to tell them," she said on ABC News' "Primetime."
Her story of the June 2002 kidnapping of Elizabeth Smart is to air Thursday on a show about five children who took heroic measures in bad situations.
Mary Katherine was 9 when she saw her sister, then 14, abducted.
Nine months later, Elizabeth was found walking in a suburb with Brian David Mitchell, a self-proclaimed street preacher who went by the name Emmanuel. He had allegedly taken her as a second wife.
The night of the abduction, Mary Katherine remembers being "sort of awake."
"I saw this guy in my room, and I'm like, 'Who is he?' " She said the man walked to Elizabeth. "He tapped her, and she's like, 'What is it?' And I guess she thought it was me."
Now 13, Mary Katherine said she does not know how Elizabeth remained calm as the man took her away. She said that when she finally crawled out of bed to find her parents, all she could say was, "Dad, Elizabeth's gone."
It was not until several months later that Mary Katherine connected the alleged abductor with Emmanuel, a homeless man who had once done work at the Smarts' house.