Then, their sister Paloma was diagnosed with cancer. Doctors told her she had a high chance of survival with chemotherapy. But in 2024, seven months later, she died - having refused the treatment.

The brothers blame their mother's anti-medicine conspiracy theories for Paloma's death at 23 - as cancer doctors tell BBC Panorama these beliefs are becoming more mainstream. Kate Shemirani has not responded directly to the allegations we raised, but she has publicly blamed the NHS for her daughter's death.

She and her ex-husband, Paloma's father Faramarz Shemirani, wrote to us saying they have evidence "Paloma died as a result of medical interventions given without confirmed diagnosis or lawful consent". The BBC has seen no evidence to substantiate these claims. Paloma's elder brother Sebastian says: "My sister has passed away as a direct consequence of my mum's actions and beliefs and I don't want anyone else to go through the same pain or loss that I have."

Both brothers say they contacted me about Paloma in the hope they could prevent other deaths, and they believe social media companies should take stronger action against medical misinformation - which the BBC has found is being actively recommended on several major sites. "I wasn't able to stop my sister from dying. But it would mean the world to me if I could make it that she wasn't just another in a long line of people that die in this way," says Gabriel. For Panorama and BBC Radio 4's Marianna in Conspiracyland 2 podcast, I pieced together how this young Cambridge graduate came to refuse treatment that might have saved her life, following an online trail and interviewing people close to her.

And I found that conspiracy theory influencers such as Kate Shemirani are sharing once-fringe anti-medicine views to millions - which can leave vulnerable people at risk of serious harm. It is getting harder to fight medical misinformation because of the prominence of figures such as Robert F Kennedy Jr, who have previously expressed unscientific views - says oncologist Dr Tom Roques, vice-president of the Royal College of Radiologists, which also represents cancer specialists. (BBC/ Getty Images)