Shirley Kordie, 33, has hypoplastic MDS and will leave her son, Blessing, four, without a mother if she is not treated.
Her brother Joseph, who lives in Ghana, is a "perfect" stem cell match but his visa application was denied due to his "financial circumstances".
The Home Office has been asked to comment.
"My life is in danger - I need to get my life back for my son. I have my little boy, and I want to live for him," the BBC reported Ms Kordie as saying.
The Anthony Nolan Trust and the African Caribbean Leukaemia Trust (ACLT) have launched a campaign to support Ms Kordie, who has been receiving treatment at Birmingham's Queen Elizabeth Hospital.
The Anthony Nolan Trust said the reason Joseph has been refused a visa is because "he doesn't earn enough money".
'10 out of 10 match'
A petition urging the government to reverse its decision has amassed about 10,000 signatures in 36 hours.
Joseph, a nurse himself, is unable to make the donation in Ghana, so coming to the UK is his only option, said the Anthony Nolan Trust.
Spokeswoman Amelia Chong said there were no alternative options for a donor on the international stem cell register.
"Her brother is a perfect, 10 out of 10 match for her," she said.
"We have reviewed all those on the donor list and he is not only the perfect match, he is the only match."
"Put simply, Shirley will die without this stem cell transplant."
What is a stem cell transplant?
Stem cells are the body's primary cells. All other cells, tissue, organs and bones develop from stem cells.
Blood stem cells, found in our bone marrow, are responsible for creating lots of different types of blood cells.
For 90 per cent of donations, cells are collected through the donor's bloodstream in a process called peripheral blood stem cell collection (PBSC).
Their blood is passed through a small tube into a machine that collects the stem cells, and then returns the rest of the blood to the body.
In order to have a stem cell transplant, the recipient must get rid of the abnormal cells using chemotherapy and sometimes radiotherapy.
New stem cells will then be infused into the recipient's blood in a similar way to a regular blood transfusion.
When the new stem cells enter the recipient's blood, they move to the bone marrow and start producing new blood cells. Over time, this leads to the development of a new immune system that can recognise and remove any remaining abnormal cells in the body.
Ms Chong added: "Her brother is ready to donate, the doctors are ready to carry out the procedure, everyone is ready but the Home Office.
"All Joseph needs is a temporary visa to undergo the procedure."
In a letter to Shirley, the Home Office said: "While I am aware of the importance of family contact and the compassionate nature of your application, I must, however, also consider your personal and financial circumstances in Ghana when addressing your application."
Shirley is currently at her home in Walsall, West Midlands, receiving regular blood transfusions.
Orin Lewis, chief executive of the ACLT, said: "The stark reality is Shirley has no other option.
"The search for an unrelated donor is made difficult due to her African heritage, which means she is three times less likely to find a perfect match." Read Full Story