Alice said the time was "especially tragic" for the family at the moment - Ms Mahy's younger brother, Frank, died suddenly on Saturday.
Mr Mahy, younger than the author by six years, had been battling Alzheimer's disease for a number of years.
Alice did not think Ms Mahy knew about her brother's death before she died.
Many of Mahy's fans emailed the Herald to share their memories of her books.
"I remember her coming to our tiny [32 children] school in Waikeria to read to us 24 years ago, wrote reader JCGC..
"She was so much fun and I have never forgotten that day. What a wonderful lady she was."
She wrote her first story when she was 7, and won numerous awards and honours for her contribution to New Zealand and children's literature.
"Margaret Mahy was a national treasure, her stories will live on through future generations. What an amazing woman," wrote Bex Carr
"My granddaughter loves the book A Lion in the Meadow - and I am reading from the same book that I read to her mother! I have emailed with Margaret from time to time, which I shall print out and treasure," wrote Coralie Myer.
Ms Mahy wrote more than 120 books which have been translated into 15 languages.
She has also won many of the world's premier children's book awards, including the Carnegie Medal and the Hans Christian Andersen Award.
A Lion in the Meadow, published in 1969, started her international career and she became a full time writer in 1980.
"It was one of those romantic things that happen," Mahy said of her own discovery.
