Frances Shand Kydd, 68, is known to have a rapidly progressing brain disease, but a statement issued by Clarence House on behalf of her grandsons Princes William and Harry simply said: "This is a private family matter and not something we would comment on."
Her son, Earl Spencer, and his sister, Lady Sarah McCorquodale, arrived at the hospital in the small Scottish town of Oban Thursday.
Her parish priest, who has visited her several times in the hospital, said the devout Catholic was "on the mend," The Scotsman reported.
Shand Kydd has rarely been seen in recent weeks outside her home on the remote Scottish island of Seil, but is always in a wheelchair.
It was unclear if she was aware of the controversy swirling around the United States over the CBS network's decision to air video footage of her daughter dying in a Paris car crash.