According to onlookers, Harriott, 65, saved her from drowning when she tripped on Monday morning.
He was pictured checking on his sibling with other passers-by as she lay on the ground next to the garden pond feature in the grounds of Chelsea Hospital, west London.
He later tweeted: “Just a big thank you to everyone who helped my sister when she had a fall at the Chelsea Flower Show this morning. She’s doing fine and all is well xx”
A group, also including DIY SOS presenter Nick Knowles, had been admiring the BBC’s display when the incident took place.
Guests said the woman ingested a large amount of water and had to be pulled out by celebrities who were at the Our Green Planet and RHS Bee Garden exhibit.
Paramedics arrived shortly after and she was taken to hospital but was well enough to go home later.
One guest told The Sun: “The garden was packed with lots of people and I think the woman must have fallen into the pond or tripped over and ended up in the water.
“I think she nearly drowned. She was pulled out by Ainsley Harriott and some other celebrities before they laid her down on the floor. She was coughing up lots of water.”
Knowles, with his girlfriend Katie Dadzie, was seen watching on as Harriott aided his sister.
Matt Peskett, a Royal Horticultural Society volunteer who saw the accident, told The Daily Telegraph that he was scheduled to show visitors around the garden during the public opening on Tuesday, but now officials have put barriers in place which could limit the tours.
Mr Peskett said: “There were 30 or 40 celebs out and they were all drinking champagne.
“And then all of a sudden one of the guests stepped backwards and fell in. They fully fell in, not just a foot.
“I hope they don’t keep it like this tomorrow – it would be a shame.”
An RHS spokesman said: “We are investigating an incident that occurred on a garden at RHS Chelsea Flower Show today.
“The guest was quickly attended to by on site medical professionals and we are pleased to say is now home and well.”