Jo Whiley missed her evening radio show after her sister, who has learning difficulties and diabetes, was admitted to hospital with coronavirus.
The broadcaster, 55, said on Twitter that her sister, Frances, 53, had become “very poorly” since testing positive following an outbreak at her care home in Northamptonshire.
Whiley said she felt “very scared” at the news, and that pop singer Will Young would be standing in for her in her regular BBC Radio 2 slot on Thursday.
She wrote on Twitter: “I can’t do my @BBCRadio2 show this evening. My sister Frances is v poorly in hospital with Covid. I don’t feel shiny or happy tonight, I feel very scared. However I’ll be listening to @willyoung who I know will light up our kitchen in the depths of our darkness.”
In a second tweet, she added: “Ps. Things we’ve learnt. Get yourself an oximeter- they are vital for exposing dangerously low oxygen levels which you can have even though u feel fine with Covid.”
Whiley has previously questioned why she was offered the vaccine before her sister, who has the rare Cri du Chat genetic syndrome.
Speaking on BBC Radio 4’s Today programme on Tuesday morning, Whiley said the care home where Frances is a resident had suffered an outbreak of Covid-19 last week.
She said the effect on her sister’s mental health had been “quite extreme”, and that Frances had been unable to see her parents.
Whiley said she had, for the first time, refused to take calls from family members.
More than 16 million people in the UK have received their first dose of a Covid vaccine.
The NHS initially targeted the top four priority groups, including people over the age of 70 and health and care staff, aiming to offer the jab to everyone in this group by mid-February.