Martha Williams, 26, was supposed to travel to the UK to donate stem cells to 24-year-old May Brown but was denied visa.
The Home Office reversed a decision not to issue Ms Williams a visa after more than 60,000 signed an online petition.
The transplant was deemed a success in March. But the African Caribbean Leukaemia Trust (ACLT) said Mrs Brown died on Friday following a relapse.
It added she passed away “surrounded by her loved ones”.
The operation was postponed from December to January after Mrs Brown, from Weymouth, became unwell with a virus and required further chemotherapy, ACLT said.
However, ACLT co-founder Beverley De-Gale said Mrs Brown relapsed in April.
“The leukaemia came back and it came back very aggressively.
“The doctors tried again to get May back into remission but sadly the leukaemia was too strong.”
She added she felt the delay in Ms Williams getting a visa may have contributed to Mrs Brown’s death.
“You never know with any cancer what way it’s going to go but any delay is never going to help the situation.
“I sadly have to say that could be a possibility but I am not a medical expert.”
Mrs Brown was diagnosed with leukaemia in July 2015.
She had been told her only chance of survival was an urgent stem-cell transplant.
Her sister Martha was a “10 out of 10” tissue match.
ACLT said Ms Williams was initially refused a UK visa because her teacher’s salary of £222 per month was too low.
Mrs Brown is survived by her husband Mike and two-year-old daughter Selina-May.