Jo Whiley said she was living a nightmare when she was offered the coronavirus jab before her vulnerable sister.
The BBC radio star gave a heartbreaking update that her sister Frances — who has a rare genetic syndrome called Cri du Chat — has since tested positive for Covid.
Frances, 53, has been staying at a care home that suffered an outbreak of Covid-19 last week.
Emotional Jo, 55, voiced her frustration at "living through a nightmare" amid the coronavirus pandemic during her chat on BBC Radio 4's Today programme.
She said: "Oh my God, I can't tell you how frustrating it is and how horrendous it is.
"It is the stuff of nightmares at the moment. I feel like I am living through a nightmare.
"All weekend it has been awful — really, really difficult. It has been hard for my parents, it has been hard for everyone in the care home, and it continues.
"And then, ironically, I got a message to say I was due to have my vaccine before my sister who has got learning difficulties and underlying health conditions. Go figure."
Jo reached out to her followers to update them on her sister's Covid battle.
The radio sensation lamented it was her "worst fear... with the vaccine so close" but she said her sister had been OK so far.
She tweeted: "Feel like I’m in a terrible film with bad plot twists. 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 crossed."
"In a heartbeat", Jo had no qualms admitting she would give up her Covid-19 vaccine for her sister during her Radio 4 interview.
She said: "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."
Although she "failed to understand" why she had been called for the vaccine, she concluded it must be because she acts as a carer for her sister.
"I fail to understand, to be honest with you," she said. "Myself, my parents and the home have done everything we can to try and facilitate the vaccine coming in to the people who need it the most.
"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'."
