Isa said the singer was still touring and performing around the world right up to December 2023 when he received the diagnosis.
The diagnosis came after the Grammy winner suffered a fall and showed other concerning symptoms such as memory loss, random headaches and falling out of his chair.
Bolton had two surgeries on his brain and underwent both radiation and chemotherapy treatment.
Isa said her father stopped performing and has remained at home to focus on his healing.
Bolton is lovingly cared for by daughters, Isa, Taryn and Holly.
Bolton undergoes MRI exams every two months to monitor his brain and to check the tumour has not returned, according to People.
The Glioblastoma Foundation said the disease has a recurrence rate of 90%.
Bolton’s daughters described the different role each plays in his care.
“We each help to honour him in various ways,” Isa said in the AARP interview.
“One of us, for instance, is very fastidious in making sure that all of the daily care… The Is are dotted and the Ts are crossed.
“One of us is worrying more about companionship and his spirits and what type of mood he’s in.
“And then we’re all kind of collectively working on managing various business and professional aspects that need to be taken care of day to day.”
Soul singer Michael Bolton had a top 20 hit in New Zealand in 1993 with ‘Said I Loved You… But I Lied’.
The Daily Mail said the sisters acknowledged they had experienced “caregiver fatigue”.
“The pressure, the burden is overwhelming sometimes,” Isa said.
Taryn added, “It’s important to be good to yourself so that you can give the best care to your loved ones”.
The sisters said they had shared more meals with him in the last two years than in their whole lives.
The How Am I Supposed To Live Without You singer said in a statement to AARP: “This past year has tested me in ways I never imagined. My daughters have been constant in a way that grounds me”.
In an interview with People in April, “I want to keep going, I feel there’s still a lot to do on the fight side”.
“I got a title for a song: ‘Ain’t Going Down Without a Fight’.”