The man said he was "angry" when his sister told of the abuse after he confronted her about her alcohol problem in her 20s.
Harris, 84, listened from the dock at London's Southwark Crown Court where he is on trial on 12 charges of indecent assault between 1968 and 1986 - seven relating to the woman in question.
During questioning from prosecutor Sasha Wass QC, the witness said he asked his sister what was causing her to drink regularly to excess.
"That's when she confided to me that she'd been sexually abused," he said.
Asked whether she said who the man was, the witness answered: "She did. She said Rolf Harris."
Harris denies all charges. The trial continues.