The X Factor star cancelled all public appearances and performances last week after he was diagnosed with acute exhaustion.
Arthur has received much criticism in the past few weeks, after using a homophobic slur in a diss track on Twitter.
Speaking to the Sunday Mirror with Arthur's blessing, Sian said: "We've been so worried about him this week, he's been diagnosed with depression and anxiety attacks.
"He put it really simply to me. He just said, 'Fame has mentally broken me'."
Sian revealed that her brother has struggled with his quick succession into the spotlight, pointing out that he turned down media training after winning The X Factor last year.
She said: "Sometimes I wish he'd never won X Factor.
"Despite finally having what he's always wanted, he says he feels like a caged animal surrounded by yes-men and is really lonely.
"He turned down media training and I think he has really struggled with this new world."
Sian, who is two years older than the 25-year-old singer, urged people to "move on" and end the "hate campaign" targeted at her brother.
She said: "It was the wrong word to use, no excuses - but he apologised immediately and the hate campaign against him has gone too far now. He made a mistake and he's so sorry about it that he's making himself ill with worry.
"He's human and people need to move on from a mistake that he made that he's deeply, deeply sorry for."
