Robert Ainsworth said hepatitis C prevented him from becoming a donor.
Nearly a week after the operation, Glenda Ainsworth said last week that she was still in tremendous pain. But she said she would do it all over again.
"On Thursday, I thought I was going to die," she said. "It doesn't really concern me. I saved him. I'd give him my last one if I had to."
A spokeswoman for the University of Maryland Hospital's pediatric intensive care unit in Baltimore said that Adam was listed in serious but stable condition Monday. Medical experts say that is normal for a patient who has just received a transplant.
Born seven years ago with kidney problems and partial deafness, Adam has struggled with health problems all his life.
Robert Ainsworth, 39, said the boy's mother denied he was the father. But he always suspected Adam was his son, and when he saw Adam's picture last year, he knew for sure.
A blood test confirmed paternity, Robert Ainsworth said, and he began to fight for custody.
Washington County Circuit Court Judge Darrow Glaser awarded temporary custody to Ainsworth last October, court records show. He has a court date in April to determine permanent custody, he said.
Robert Ainsworth said he feared his son was going to die in February.
"He was confused. He didn't know nobody," he said.
Because of Adam's medical ailments, Robert Ainsworth has been traveling to Baltimore three times a week. Because of his work schedule, he said he has been awake for 48-hours at a time.
Adam still faces a long road, but Robert Ainsworth said he is confident that all will go well.
"Things are getting a lot better," he said.
Ainsworth said a big concern is paying the bills. He said his insurance will not cover his son because of a pre-existing condition.
Even with a new kidney, Adam needs to take multiple medications that could cost thousands of dollars a month, Ainsworth said.
But Robert Ainsworth said he is prepared to do whatever it takes to ensure his son's health.
"I don't care if I got to work three jobs," he said. "That's what I'll do."
While Robert Ainsworth praises his sister for her sacrifice, she said her brother deserves most of the credit.
"A lot of people would have turned their back on this baby," she said.