The sister donated her kidney, even though their blood types didn't match.
It's an operation that would've been impossible just a few years ago.
There have been just 50 of these transplants performed in the United States.
Barbara Cullen is walking, talking proof that what was once considered impossible is now reality.
"I feel like a different person than I did before!" said Cullen.
The Silver Spring woman had been on the kidney transplant waiting list for three years.
She was a professional chef.
She had lost her taste.
Everything that was enjoyable was being lost.
Marybeth Mullen wanted to donate one of her kidneys, but while she and her sister were genetic matches, their blood types didn't match.
Normally, that means a transplant isn't possible.
But Doctor Keith Melancon at Georgetown explained they could do an incompatible kidney transplant.
First, doctors had to basically clean Barbara's blood.
Doctor Melancon said, "We pulled away the stuff in her body that would attack the... We gave her proteins to block her antibodies that would normally attack the organ."
Six weeks post-surgery, and Cullen's about to head back to work.
She says she's forever grateful to her sister.
Cullen said, "she gave up a perfectly good kidney to save my life."
Both women are thrilled Cullen has her energy back.
She'll need it - her first grandchild is due next week.
And thanks to her sister, Cullen will be here for it all.
"I've always known she's been a good sister, but she's a lifesaving sister," said Cullen.
Doctor Melancon says there is a higher risk of rejection with incompatible transplants but patients who don't have a major episode of rejection within the first two weeks, typically go on to lead normal, healthy lives.