On 14 April, Ms Barrow will sell her brother's bullet-ridden and bloody shirt, along with some of his other personal belongings, to raise money to move his grave.
"Bonnie begged her mother to bury them together, and her mother promised all the time that she would," Ms Barrow told the Dallas Morning News.
"But then she didn't do it. I guess she thought Clyde had taken Bonnie off and got her killed. You know how mothers feel."
As celebrated as OJ Simpson in their time, Bonnie and Clyde were gunned down in an ambush near Gibsland, Louisiana, in May 1934, aged 23 and 24. More than 16,000 people lined up to see their publicly displayed bodies.
Their deaths ended a two-year crime spree in which the Barrow gang robbed banks and shops across the South-west and Mid-west, killing at least 15 people. They were buried in separate cemeteries in Dallas.
The San Francisco auction house Butterfield and Butterfield,will take bids on the shirt, Clyde's pocket watch - still running - and a series of original Bonnie and Clyde snapshots from the Barrow family album, including some of them posing with their guns.
Spokesman Levi Morgan said the items are valued between $56,000 and $76,000 (pounds 35,000 and pounds 48,000).
Other memorabilia have been sold in previous auctions, he said, including the car in which they died and which contained 15 firearms.
Clyde bought the shirt in Indianapolis while he was on the run, and wore it when he was ambushed by Texas Ranger Frank Hamer, the man who tracked down the couple, and six other men. It has pearl buttons and more than 30 bullet and buckshot holes. The shirt was removed by a mortician and passed on to his father. It is expected to sell for up to $45,000.
Also on sale is one of Clyde's first rifles, which he played with as a child, pretending to be Jesse James.