Twins Michelle and Lavinia Osbourne have always shared a special connection.

But when Lavinia clicked on an email with results of an at-home DNA test in September 2022, she was filled with a sense of dread.

"Maybe subconsciously I knew," she says.

Her test results revealed something astonishing: non-identical twins Lavinia and Michelle don't have the same father.

They were conceived naturally, grown together in the same womb, and born to the same mother within minutes of each other - but they are half-sisters.

Michelle and Lavinia, 49, exist because of an incredibly rare biological process called heteropaternal superfecundation. For it to happen, a woman must produce more than one egg during the same cycle, the eggs must be successfully fertilised by sperm from different men, and the resulting embryos must survive the pregnancy.

Only around 20 cases have ever been identified worldwide. After months of researching their story for the BBC Radio 4 series, The Gift, I've found that Lavinia and Michelle are the only set of twins with different fathers ever to be documented in the UK.

For Lavinia, the revelation was devastating. She and Michelle had shared a difficult childhood where they were passed around between homes and carers. The only stability the non-identical twins had was each other.

"She was the one thing that belonged to me, the one thing that I was certain about, the one thing that I was sure of," Lavinia says. "And then she wasn't."

But when Lavinia rang her twin to share the news, Michelle felt differently.

"I wasn't surprised," Michelle says. "I'm still in amazement that this can actually happen – it's super weird, super odd, super rare – but it makes sense."

"She had suffered abuse at the hands of [her] stepfather," Michelle says. "My mother was in and out of foster care and children's homes throughout her childhood."

Whenever the twin sisters asked who their dad was, their mum always said he was someone called James. "He wasn't in our life," Michelle continues.

Their mother was absent for much of their lives, too. When they were five years old, she got a place to study at university in London, and left her children behind in Nottingham with her best friend's mother, who the twins called "grandma".

"Grandma was strict – not very emotional, not very cuddly. The one constant I had was Michelle," Lavinia says.

As long as she had her twin, Michelle says, she felt safe.

"It was her and I against the world."

Aged 10, the girls joined their mother in London. But within a few years, Lavinia and Michelle were sent away again, to live in one of their mother's old foster homes. They couldn't understand why their mum wanted to maintain a distance from them.

"Physically and emotionally, she was always out of reach," says Lavinia.

By late 2021, their mother had early-onset dementia, and was no longer able to answer their questions. Michelle saw a photograph of James, and became more convinced than ever that he couldn't be her father.

"I just thought, you don't even look anything like me," Michelle says. "So I bought myself a kit."

If you take a DNA test, you reveal truths about your family as well as yourself. But Michelle wasn't thinking of how her results might affect Lavinia when she sent her sample off to be analysed. The results arrived on 14 February 2022 - the same day Michelle and Lavinia's mother died.

James's last name didn't feature in Michelle's paternal line - he wasn't her father.

After weeks of investigation, Michelle discovered that her father was Alex, the brother of a woman who had been friends with their mother. Michelle contacted some of Alex's family, who warned her he had struggled with alcoholism and drug addiction for years, and was living on the streets.

Michelle and Lavinia met up with a woman called Olivine, who Michelle believed was a new first cousin to both her and her twin. Michelle felt an instant connection.

"I just knew she was blood," she says. But Lavinia didn't feel the same. And when Olivine took out photographs of her family, Lavinia didn't see herself in their faces.

Lavinia decided to also do a DNA test. She didn't expect to get a different result to Michelle, but she had to do something about her growing feelings of unease that Alex's family was not her own.

"I just wanted confirmation," she says.

When she opened her results and saw the mind-boggling truth that her twin was her half sister, Lavinia was distraught – and furious.

"I was angry with Michelle for having me go through this, because I just didn't want this reality."

And there were more revelations to come. When Lavinia looked closely at her results, she saw that James wasn't her father either.

Lavinia had no interest in finding out who her dad was, but Michelle was determined to get answers. Combing through her twin's results, Michelle found Arthur, Lavinia's biological father. The twins drove to meet him at his home in West London.

"He was a little bit nervous, but he's got an exuberant character - like me," says Lavinia. At the end of their meeting, she kissed Arthur on the cheek.

"I just felt compelled, drawn to do it."

Lavinia and Arthur have been close ever since, seeing each other several times a month, often alongside Michelle.

"I feel like I've found a place to belong, and that place is with my dad," Lavinia says. He has told Michelle that she, too, can call him Dad.

At a later meeting, the twins asked Arthur what he knew about how they came to be conceived.

"He said, 'Your mother knocked on my door. She was very upset. She was crying'," Michelle says. "She went to him because she wasn't safe, and she was in shock."

Michelle and Lavinia's mother is not here to tell us exactly what happened, but Arthur says she turned to him in a time of need.

Michelle was also able to meet her own biological father, Alex, through other newly discovered family members.

"He was clearly under the influence of drugs," Michelle says.

The resemblance between them was undeniable.

"He's mine, I'm his, but I didn't feel like he's someone I need to take forward in my future with me," she says.

"I just needed to know."

The twins will never know whether their mother suspected they had different fathers.

"It must have driven her crazy," says Lavinia. "She must have seen something, felt something."

"I think in the back of Mummy's head, she knew, but she was in great denial," Michelle says.

As non-identical twins, Lavinia and Michelle have always been aware of their differences. The truths revealed in their DNA made those differences seem even more stark, at first. But they will always share a unique bond.

"We're miracles," Lavinia says. "We're always going to have a closeness that can't be broken."

"She's my twin sister," Michelle nods. "Nothing takes away from that." (BBC, excluding headline)

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