Susan Squires and her sister, Angela Latham, both from Blackpool, did not speak for three years after Ms Squires refused to consider a transplant to help her sister fight leukaemia.
Doctors treating 34-year-old Mrs Latham had scoured a worldwide register of 4 million bone marrow donors before concluding that only Ms Squires, 39, could help.
Ms Squires finally agreed in April, after she was told she need not go to hospital. She gave blood samples at her home and "had come to terms" with the operation.
But tests showed the sisters' marrow was a "partial mismatch" and Ms Squires was only as suitable as an unrelated donor.
"She was as disappointed as I was with the results of the tests. After everything we'd gone through it was not what I was expecting," Mrs Latham said.
The mother-of-two has vowed to go ahead with the operation if possible, despite warnings of complications. She is now waiting on advice from specialists at Manchester Royal Infirmary as to whether the transplant should go ahead.
"It's a case of weighing everything up. All we can do is wait."