The star famously withdrew from the music scene to return home to Cambridge after leaving Pink Floyd in 1968, followed by a short-lived solo career. He died in 2006 aged 60 from pancreatic cancer.
Due to his reported drug-related reclusiveness, Barrett became, and continues to be, a cult figure among music fans.
Rosemary said of her brother: “People ask me why Syd withdrew and I think it was a combination of too much LSD and a very eccentric creative brain.
“If you’re very tired and put a load of acid into the mix you’ve got chaos haven’t you? It was all very nasty but it was inevitable. When you put those three factors together you are going to have an explosion aren’t you?”
She added: “Syd still saw music in a light-hearted way. It was still fun, and it never was serious until it all went wrong and then it was very serious.
“Fame was the last thing he wanted and he never understood it. He never needed it because from the moment he was a child he was surrounded by people who adored him.”
Rosemary also added that after returning to Cambridge permanently in 1981 he showed no interest in Pink Floyd at all.
She said: “He just tried to put that whole thing away. If anyone called him Syd he wouldn’t answer (his real name was Roger). He wasn’t Syd because Syd was Pink Floyd. He wasn’t being clever by being reclusive, he was just being himself.
“It wasn’t contrived and he tried very hard to disappear, succeeding to some extent, but that just made some fans want to know about him even more. It’s human nature.”
She explained: “Roger was never mentally ill. He was assessed by quite a few psychiatrists over the years and they always said he’s unusual but there is no illness. There was no cure because there was no illness. He never fitted into the norm but that’s what made him so special.”
A tribute exhibition to Barrett, including paintings, photos and more runs from October 22 to November 1 in Cambridge. See Thecitywakes.org.uk for more information.