More than 12,000 children in care are not living with at least one of their siblings, the BBC has discovered.
Freedom of Information requests sent to more than 200 UK local authorities revealed that more than half of sibling groups in care are split up.
The law says brothers and sisters should be placed together if possible because it is in their best interests.
The Department for Education said where siblings cannot be accommodated together, contact should be maintained.
Latest figures show the number of children in care is rising. In England alone there are more than 78,000, which is an increase of 28% in the last decade.
Many of them will have brothers and sisters, but there is little data about the extent of sibling separation.
File on 4 submitted Freedom of Information requests to all UK local authorities with responsibilities for care, and two-thirds provided data.
Figures also showed that at least 1,375 children who were placed for adoption between April 2018 and November 2019 had been separated from birth siblings.
However, the real figure could be higher because a third of authorities did not provide data.
'I was alone'
When Sophia was little, she told her adoptive mother she had an imaginary friend. She even had a name for her - Theighan
But it turned out Theighan was not imaginary at all. She was Sophia's older sister, who had been separated from her at the age of four while both girls were in care.
The memory had simply faded over time.
"I just remember having the person there that I trusted and then I didn't and then I was alone," said Sophia.
Theighan had remained in care, but her little sister was never far from her thoughts.
"I would write birthday cards to her on her birthday and write Christmas cards to her. I wasn't allowed to send them to her because she was adopted. I wasn't allowed to talk to her in any way, so I kept all these cards hoping that I'll be able to give them to her some day," she said.
Daniel Monk, professor of law at Birkbeck, University of London, says while each case should be looked at individually, the importance of these relationships should not be overlooked.
"The evidence from some young people is they found this hugely traumatic," he said.
"Wherever possible they should be kept together and I think people will say that is what the current law states at the moment. The problem is that is easily outweighed by other considerations."
Those considerations include shortages of foster carers, the space to accommodate large sibling groups and a lack of clarity over the definition of a sibling - does it include half-brother and sisters and step-siblings?
For those separated while in care, guidance says that every effort should be made to keep them in contact with each other.
But, in practice, these arrangements can slip because of shortages of time and resources.
There is a legal duty for children in care to have reasonable contact with their parents, but siblings are not specified by statute.
Adoption can also be a risk moment for brothers and sisters in maintaining their relationships.
"When a child is adopted they are no longer considered the blood relative of their birth relatives," said Prof Monk.
"The effect of adoption is that the relationship with siblings ends in law."
Eventually Theighan managed to get a letter to Sophia after 12 years apart.
Then they made the momentous decision to meet.
"It was just like the missing piece of the jigsaw just clicked back into place. There really was no awkwardness, and it was my favourite date ever," said Theighan.
It has been three years since they met, but the two still cry together as they reflect on all the years they spent apart.
"We love each other and we get along so well. And we could have been getting on so well for 12 years and that's over a decade of not seeing each other and missed memories," said Sophia.
The Scottish government told the programme a bill going through its parliament will place a new duty on local authorities to promote contact between siblings whenever a child is taken into care.
In England, the Department for Education says it is investing a further £46m to help adoptive families open up their homes for hard-to-reach groups, including siblings.
In Wales, children in care are entitled to an advocate to help them understand their rights and entitlements.
Northern Ireland says it already has legislation to promote direct contact between siblings.
File on 4's Separated Siblings is on BBC Radio 4 on Tuesday 14 January at 20:00 GMT and available afterwards on BBC Sounds.