Emma and Taylor Bailey of the Queen Creek area died Tuesday at Seattle Children's Hospital during an operation on their shared heart.
RELATED: Website for Emma and Taylor
Family friend Ginni Christiansen said Thursday that the girls had had the surgery twice before but this time an artery burst and doctors were unable to save them.
The girls had defied medical expectations and would have turned 4 next month. They had been scheduled to undergo a risky operation to separate them later this year.
Fundraisers have been held to help the family and last year dozens of celebrities joined in with a dinner theater production in the Phoenix area.
Information from: The Arizona Republic