Marilou Danley, a 62-year-old with Australian and Philippines citizenship, arrived at Los Angeles airport on Tuesday evening after two weeks in the Philippines.
Her sisters told Australia's Seven News that they believed she had been "sent away" by Stephen Paddock so that she wouldn't "interfere with what he's planning."
On Sunday, Paddock opened fire on thousands of revelers at a country music festival in Las Vegas, Nevada, from a hotel room on the 32nd floor of the Mandalay Bay hotel.
He killed 59 people and injured more than 500 in the most deadly shooting in US history. When police stormed his hotel room, they found that Paddock had already committed suicide.