Gold Logie winning actor Samuel Johnson says his sister Connie is in palliative care and is "actively dying".
The sad news was posted on his Love Your Sister cancer fundraising Facebook page on Monday.
"Con's needed a lot of quiet time lately to try and comprehend the total headf****ery that 'actively dying' brings," Johnson wrote.
The actor said his sister had taken time out from phone calls and social media as she began her most difficult challenge yet.
"In true Connie style she's been colouring in to keep the nasty thoughts at bay," he wrote.
"She said to tell you that there's life in the old dog yet."
Ms Johnson's battle with cancer began as an 11-year-old when she was diagnosed with an aggressive bone tumour in her leg.
Now 40, she has terminal breast cancer which has spread to her lungs, liver, pelvis, spine and knee.
Samuel Johnson quit acting last year to focus on fundraising for the Love Your Sister cancer charity he set up in response to his sister's diagnosis.
He pledged to his 380,000 Facebook followers to raise $10 million before taking another acting job.
Johnson also travelled around Australia on a unicycle to raise funds for cancer research.
More than 7000 people have commented on Johnson's social media post, many thanking the siblings for their hard work.
"Connie, Sam you created the village. You created a community, and that community are all thinking of you. Feel our love," Susan Munroe wrote.
"Some people are here for a little while to touch hearts and change the world. What an incredible woman she is," added Wendy Gh.
"Damn this post just broke my heart. How does one prepare for such an ordeal? How does one’s family deal with the enormity? F*** you cancer!!!!," Melissa Felton said.