The youngest member of sibling group the Osmonds suffered a stroke in December during a pantomime performance of Peter Pan in Birmingham, where he was playing Captain Hook.
Merrill Osmond, the group's former lead singer and bassist, told PA that a return to the stage for his brother was "off the cards".
The 66-year-old said Jimmy - who hit number one in 1972 with solo record Long Haired Lover From Liverpool - would "hopefully" spend the next year with his family.
However, he said the illness was "a big enough disaster" that he doubted the 56-year-old would perform live again, adding: "I think they are off the cards. I think they are off, brother.
"We used to be called the Osmond Brothers. Now it's down to Brother Osmond.
"It's gone. The amount of health issues along (the way), it's taken everyone pretty well down, except for Donny, Marie and myself.
"The music is still there. The excitement is still. The definite interest is still there from a global standpoint.
"Until something happens to me I'm going to keep charging ahead, doing the things I love doing, and that's music."
The Osmonds' string of hits, including Crazy Horses, propelled the US family group to worldwide stardom.
Merrill, the eldest of the Osmond men, said his brother was still refusing to see anyone but family at home in the US.
He said: "He definitely needed space - and he still does. He doesn't want to meet with anybody. He will meet with family.
"The rest of the time it is basically family, and all the space he needs to get better.
"I see him often. We are very close brothers. We've tried to stay around the details of it because he has asked us to not to get into details.
"But it's a big enough disaster that he just needs to have his life and live it the way he wants.
"We are all trying to respect that - and do what we can do to help him while we can."
Merrill Osmond plays in Leeds at the Queens Hotel on August 9 and the Royal Armouries on August 10, before returning to the UK in December for three more dates.