The sisters were convicted in 1994 of leading two men into an ambush in central Mississippi the year before. Three teenagers hit each man in the head with a shotgun and took their wallets -- making off with only $11, court records said.
Jamie and Gladys Scott were each convicted of two counts of armed robbery and sentenced to two life sentences.
"I think it's a victory," said the sisters' attorney, Chokwe Lumumba. "I talked to Gladys and she's elated about the news. I'm sure Jamie is, too."
Civil rights advocates have for years called for their release, saying the sentences were excessive. Those demands gained traction when Barbour asked the Mississippi Parole Board to take another look at the case.
The Scott sisters are eligible for parole in 2014, but Barbour said prison officials no longer think they are a threat to society and Jamie's medical condition is costing the state a lot of money.
Lumumba said he has no problem with the governor requiring Gladys to offer up her organ because "Gladys actually volunteered that as part of her petition."
Lumumba said it's not clear what caused the kidney failure, but it's likely a combination of different illnesses over the years.
Barbour spokesman Dan Turner told The Associated Press that Jamie Scott was released because she needs the transplant. He said Gladys Scott will be released if she agrees to donate her kidney because of the significant risk and recovery time.
"She wanted to do it," Turner said. "That wasn't something we introduced."
Barbour is a Republican in his second term who has been mentioned as a possible presidential contender in 2012. He said the parole board agreed with the indefinite suspension of their sentences, which is different from a pardon or commutation because it comes with conditions.
An "indefinite suspension of sentence" can be reversed if the conditions are not followed, but those requirements are usually things like meeting with a parole officer.
The Scott sisters have received significant public support from advocacy groups, including the NAACP, which called for their release. Hundreds of people marched through downtown Jackson from the state capital to the governor's mansion in September, chanting in unison that the women should be freed.
Still, their release won't be immediate.
Mississippi Department of Corrections Commissioner Chris Epps said late Wednesday that he had not received the order. He also said the women want to live with relatives in Florida, which requires approval from officials in that state.
In general, that process takes 45 days.
Mississippi NAACP President Derrick Johnson said the Scott sisters' release will be "a great victory for the state of Mississippi for two individuals who received an excessive sentence" and he has no problem with the kidney donation requirement because Gladys Scott volunteered.
"I think it's encouraging that she's willing to share a kidney so her sister can have a better quality life," Johnson said.
National NAACP President and CEO Benjamin Todd Jealous said the suspension of the sentences represents the good that can come with the power of governors.
"It's again proof that when people get engaged, keep the faith, we can win," Jealous said.