Jamie and Gladys Scott were released on the condition that Gladys Scott donate a kidney to her sister within a year.
Now Jamie Scott, 38, says doctors told her she has to lose 100 pounds before the operation.
Gladys Scott, 36, listed at 4 feet 9 inches tall and 184 pounds, has to lose 60 pounds and stop smoking.
A personal trainer has been working twice a week with the sisters. They've also been taking aerobics classes.
The two had served 15 years of their life sentences for armed robbery when they were released Jan. 7.
Mississippi Gov. Haley Barbour hasn't said whether he will send the sisters back to prison if the transplant doesn't happen.
The Associated Press contributed to this report.