"She has a recurring ulcer," Lisa Ling said. "And we know that she has been allowed to receive some medication. But we, we know that her doctor is very concerned. He has written a letter, appealing to the North Korean government to at least allow her to see a physician. And as we all know, ulcers are exacerbated by stress."
Lee's husband, Michael Saldate, left to take care of the couple's 4-year-old daughter, said he has told his daughter Hannah only that her mother is at work.
Earlier this summer, Lisa Ling told ABC News' Bob Woodruff that she dreamed of the moment of her sister's release.
"It's a scene that I've kind of replayed over and over in my head. I just hope it comes soon," an emotional Ling said.
Clinton would be the second former U.S. president, after President Carter, to visit the reclusive communist state. Carter traveled to Pyongyang in 1994, when Clinton was in office, and met with then-North Korean leader Kim Il-sung, father of the current leader Kim Jong-Il.
Carter's visit led to a breakthrough deal in which North Korea was to dismantle its nuclear program in exchange for oil. The deal -- the so-called 'Agreed Framework' -- fell apart after President George W. Bush listed North Korea as part of the 'axis of evil'.
Clinton was widely expected to make a state visit to Pyongyang during his time in office, but it never happened, as his administration at the time chose to focus on Middle East issues.
ABC News' Kate Snow, Sunlen Miller and Yunji de Nies contributed to this report.