Martha Williams, 26, travelled to the UK to donate stem cells to 24-year-old May Brown.
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."
King's College Hospital in London, where Mrs Brown received her treatment, refused to comment on the matter.
However, a spokeswoman said staff were "saddened" by her death, adding: "Our thoughts are with her family and friends at this difficult time."
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 Â£222 per month was too low.
Mrs Brown is survived by her husband Mike and two-year-old daughter Selina-May.
Paying tribute to his wife, Mr Brown said: "May will forever be remembered in our hearts.
"She was a strong, beautiful, supportive, wonderful wife and mother. Selina and I will truly miss her."
In a statement, the Home Office said Ms Williams' visa was issued on compassionate grounds on 21 October.
A spokesman said the visa was initially refused on 5 October because she did not meet the requirements of the immigration rules.
A woman with leukaemia at the centre of a campaign to allow her donor Nigerian sister to come to the UK has died.