https://en.wikipedia.org/wiki/June_and_Jennifer_Gibbons

June and Jennifer Gibbons were identical twins born to a Caribbean couple in 1963. The family moved to Haverfordwest, Wales, shortly after the children's birth.

From the time they learned to speak, June and Jennifer had a particular high-speed patois that made it difficult for people to understand them. Being the only black children at their school, they were frequently bullied by their peers. The bullying led to their special language becoming even more irregular, to the point where no one could understand what they said.

At 14 they were sent to seperate boarding schools, to encourage them to thrive on their own. Both girls became catatonic and were quickly reuinted. Following their renuion, they spent several years locked in their room, playing with dolls, putting on operas for one another, and writing.

They wrote several short stories that they self-published. The stories tend to revolve around criminality and sex. After the stories failed to garner any attention, they left the confines of their room and started living in the real world. They continued to speak with no one but each other. They started committing crimes such as arson, theft, and assaulting one another. A judge ruled that they need to be committed, and they were sent to Broadmoor Hospital, where they remained for 14 years.

In 1993 it was decided that the twins would be transferred to Caswell Clinic, a lower-security facility. When the twins arrived at Caswell, Jennifer was found to be unresponsive. She was rushed to the hospital but later pronounced dead. The cause of death was acute myocarditis or a sudden inflammation of the heart. No drugs or poisons were found in her system.

A few days after Jennifer's death, June was reported to say, "I'm free at last, liberated, and at last Jennifer has given up her life for me."

June immediately began speaking to others and integrating into society. She was released from the mental hospital and from then on lived (and continues to live) a so called "normal life."

The central question of the strange silent twins remains -- what exactly happened to Jennifer Gibbons? Did she commit suicide? Did June murder her? Was she coerced into suicide? How did she commit it?

The answer most likely lies in the psychology of the twins relationship. Although clearly co-dependent, they also had a relationship filled with animosity. June, the firstborn of the two, wrote of her sister,

"Nobody suffers the way I do, not with a sister; with a husband, yes; with a wife, yes; with a child, yes, but this sister of mine, a dark shadow robbing me of sunlight, is my one and only torment.”

Jennifer considered her "older" sister June to be her superior in every way. June knew this and wrote,

"She wants us to be equal. There is a murderous gleam in her eye. Dear lord, I am scared of her. She is not normal … someone is driving her insane. It is me.”

For her part, Jennifer wrote of her sister,

We have become fatal enemies in each other’s eyes. We feel the irritating deadly rays come out of our bodies, stinging each other’s skin. I say to myself, can I get rid of my own shadow, impossible or not possible? Without my shadow, would I die? Without my shadow, would I gain life, be free or left to die? Without my shadow, which I identify with a face of misery, deception, murder.”

Whatever the true nature of their relationship, it seems June at least has found peace. On Jennifer's gravestone is a poem that June wrote for her. It reads: “We once were two/We two made one/We no more two/Through life be one/Rest in peace.”

Some info taken from this article: http://www.the-line-up.com/the-silent-twins-gibbons/