June and Jennifer Gibbons were a set of identical twins born in 1963. Their parents were West Indian immigrants who had moved to Wales. The twins grew up inseparable, and they had speech impediments that made it difficult for them to communicate with people outside their family.
They were bullied in school, and they retreated more and more. Eventually, their language became so idiosyncratic that they could only understand each other. When they grew up they tried writing for a living, which didn’t go so well.
Desperate for attention and fame, they committed a series of petty crimes, including arson. This got them committed to Broadmoor Hospital, a high security mental facility, where they stayed for 14 years.
During their stay at the hospital, they became convinced that it was necessary for one twin to die, and eventually Jennifer agreed to sacrifice herself. Within hours of being released, Jennifer died of inflammation of the heart. No evidence of drugs or poison was found in her system.
Read more about these twins at the source.