Bailey began working as Michael’s personal assistant in 1984, joining him in the year Wham! had their first number-one hit with Wake Me Up Before You Go-Go.
She remained his PA for 20 years, and her brother told News she avoided the limelight and guarded the pop star’s privacy, despite the attention of tabloid journalists hoping for stories.
“I think she was one of the few people George trusted and whose opinion he valued,” Bailey said.
“She was always honest and straight with him. I guess that’s why she stayed working for him for so long through the very, very good and not so good times.”
The guitar now going under the hammer was “a gift” to his sister because the singer “thought so much of her”, he said.
The instrument had a key role to play in Michael’s reinvention from teen pop idol to serious solo artist.
In the 1987 video for Faith, Michael – in ripped blue Levi’s jeans, white T-shirt and black leather biker jacket – strummed the vintage acoustic guitar that he reportedly picked up in a shop in Los Angeles shortly beforehand.
Guitarist magazine recently billed it as “arguably the most famous Gretsch Synchromatic in the world”.
It has an estimated price tag of between £60,000 and £120,000. Other less significant but more personal items have already exceeded their estimates.
A Filofax from the 1988 Faith tour with Siobhan’s handwritten notes and annotations had a guide price of up to £600 and a current bid of £1,750.
Other items include a note written “with love and admiration” from Michael and Ridgeley, an official Wham! photograph signed “To Siobhan (Special…)”, and an invitation to Ridgeley’s 21st birthday party.
The auction also includes the gold disc and platinum CD presented to her for sales milestones achieved by Michael’s music.
“I know she was proud of the various personalised gold and platinum discs she got, though she was very modest about it all,” Bailey said.
He said “she would never have consented” to the sale of these items, adding that the auction “breaks my heart”.
“I think she’d be devastated that something as personal as her Filofax and notes from George are up for grabs. It’s just so tawdry and contrary to how she was,” he said.
Siobhan died suddenly and unexpectedly of natural causes at the age of 56. According to her brother, she wanted the possessions to “have remained with her family and we would never have sold”.
He accused Belcher of deciding “12 years after my sister’s death to cash in”.
Bailey acknowledged that Belcher has a legal right to sell the items but called it “disingenuous” that the sale was being promoted in her name.