Malcolm Young, aged 61, was diagnosed with dementia earlier this year, and is believed to be receiving treatment in a Sydney care facility.
In an interview with ABC, Malcolm’s brother spoke about first noticing something was wrong while the band was recording their Black Ice album back in 2008.
"When we were writing songs together, me and him, then, it was noticeable. It was just strange things, you know? I mean, memory things," he said.
The band announced Malcolm’s retirement from the legendary rock group in September, with Angus confirming that Malcolm could no longer remember his own songs.
The remaining members of AC/DC then had to make a decision about whether AC/DC would continue making music.
"To me, it was always up to Angus really," said lead singer Brian Johnson.
"If Angus turned around and said, ‘I can’t do it without Malcolm’, I would have understood perfectly."
In the end, Malcolm decided for them, urging the remaining members to 'keep making music'.
"He just said to keep doing it. That’s the way he talked – [he was a] straight shooter. So we did, and it was great," said Johnson.
Malcolm and Angus’ 58-year-old nephew, Stevie Young, has stepped in to replace the guitarist ahead of AC/DC's 2015 world tour.
The band are also set to release a new album, Rock or Bust, this week.