The defining image of Hurricane Harvey shows elderly rest home residents sitting in a lounge half-submerged in floodwater.
Susan Bobrick of Houston told the New York Times that her sister lives at La Vita Bella, the rest home depicted in the now-viral image.
Ms Bobrick said relatives of residents were contacted by an employee a couple of hours before dawn on Sunday.
She was told that water had begun to seep into the single-storey building, but that emergency responders had instructed staff and residents to stay put.
The water continued to rise for the next few hours.
Tim McIntosh posted the photo after his mother-in-law Trudy Lampson, the nursing home owner, sent it to him.
Mr McIntosh and wife Kim said they received no other contact from Ms Lampson, and posted the photo to try to get help to the rest home.
"We thought the situation was dire."
The photo was retweeted almost 5000 times.
Several hours later McIntosh tweeted that the residents had been rescued.
A follow-up was posted by CBS News with a photo showing the residents safe and dry after being evacuated.
Death toll rises
Hurricane Harvey's death toll has risen to nine, with Texas officials expecting the number to increase in the coming days.
Heavy rain continues to lash Houston, with the risk of flash flooding still very high.
The National Hurricane Center has issued a statement warning of "ongoing catastrophic and life-threatening flooding" from Texas to Louisiana, with isolated storm totals "as high as 50 inches'.
So far 34 trillion litres of rain have fallen on the city - the equivalent of 13.6 million Olympic swimming pools.
About 120,000 people are without power as night falls.
Houston's major evacuation shelter is full and will have to start turning people away.
A Red Cross spokesperson told the Guardian around 5,500 evacuees have reached the George R. Brown Convention Center, exceeding the shelter's capacity of 5,000.
Trump's reaction
US President Donald Trump and First Lady Melania will visit Corpus Christi, Texas, to receive a briefing on relief efforts.
The president has been criticised for his unrelated tweets as the storm unfolded, which included a criticism of Mexico and a plug for a friend's book.
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