Jo Whiley says she's had her "worst fears realised" after her younger sister, who has learning difficulties and diabetes, tested positive for coronavirus.
The radio DJ revealed on Twitter on Monday she had been offered a COVID jab before Frances and said she "desperately" wished her sibling had been offered one first.
She had earlier told her followers it had been an "anxious weekend" for the family after an outbreak was confirmed at the care home where Frances, 53, is a resident.
On Tuesday, the 55-year-old said her sister had tested positive for coronavirus.
"Feel like I'm in a terrible film with bad plot twists," she wrote in a post. "Late last night I got a call to say that Frances, my sister, had tested positive & has COVID.
"Our worst fears realised after keeping her safe for a year & with a vaccine so close… she's ok so far.. Everything's crossed."
More than 15 million people in the UK have been given their first dose of a COVID vaccine, with the NHS initially targeting the top four priority groups – consisting of those aged 70 and over and those clinically most vulnerable.
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In an interview with Radio 4, Whiley said it was for this reason that she wasn't sure why she got the call before her sister – although said it could be because she is classed as her carer.
"I fail to understand, to be honest with you," she said.
"She is in tier six but she also has quite bad diabetes, which in my understanding puts her in tier four because she has an underlying health condition, so I would have thought that she would have been vaccinated, but that hasn't happened.
"And I suppose what I am doing is just wanting to speak up for people like Frances, people who live in her care home, who have been overlooked, because this happens so often.
"People with learning difficulties are neglected. They haven't got a voice, they haven't got anybody there. Just badgering everybody saying 'What about me? Help me out here'."
She added: "And I would give up my vaccine in a heartbeat if I could for my sister and any of the residents in her house to have their vaccine. It just does not feel right."
Meanwhile, Edel Harris, the chief executive of learning disability charity Mencap, said it was "encouraging" to see the government reach its 15 million milestone, but stressed there were too many people with disabilities still waiting.
She said: "People with a learning disability are six times more likely to die from COVID-19 than the rest of the population yet those with a mild or moderate learning disability aren't prioritised at all. We urge the government to include everyone with a learning disability in group six urgently – it is not too late.
"It's unacceptable that within a group of people hit so hard by the pandemic, and who even before COVID died on average over 20 years younger than the general population, many are left feeling scared and wondering why they have been left out.
"The Joint Committee on Vaccination and Immunisation (JCVI) and government must act now to help save the lives of some of society's most vulnerable people by urgently prioritising all people with a learning disability for the vaccine."