If confirmed, the sisters' deaths would represent the first human-to-human transmission of the illness. Until now, health officials had concluded that human victims in Vietnam and Thailand were contracting the virus through direct contact with infected fowl.
The deaths of the two women bring to 10 the number of confirmed bird flu deaths in Vietnam. There have been two confirmed deaths in Thailand from the virus, which has hit 10 Asian nations and killed millions of chickens and ducks.
"The investigation has not been able to conclusively identify the source of infection for the two sisters," the World Health Organization said in a statement issued by its Hanoi office. "However, WHO considers that limited human-to-human transmission, from the brother to his sisters, is one possible explanation."
Although the number of human victims of avian flu has been relatively small, health experts fear that if the virus gains a foothold in the human population, it could mutate and develop the ability to spread easily among people, potentially killing millions worldwide.
The Los Angeles Times is a Tribune Publishing newspaper.