The family of the patient, Helen Huynh, said she is struggling to fight acute myeloid leukemia, a disease she was diagnosed with in February.
The only hope of saving the 60-year-old woman is through a stem cell transplant, according to the family's Go Fund Me page set up to help with legal and medical bills.
The procedure requires a genetic match to Helen, and her youngest sibling, Thuy, is a rare 100 percent match.
Thuy's three previous visa applications were denied by the US Consulate in Ho Chi Minh City.
Today, Helen’s husband told the BBC that his sister-in-law had just informed them that she had been granted the visa.
In a previous refusal letter, the consulate in HCM City stated that Thuy had failed to offer the necessary evidence that she would leave America once her visa had expired, The Washington Post reported.
As a last effort, Helen's family hired an immigration lawyer to file for humanitarian parole for emergency entry into the country.
According to the BBC, Helen’s oldest daughter set off for Vietnam today to bring Thuy back to the US for the procedure.