I think Geoff is referring to the case of Constance Kent who was accused of killing her 3 year old half brother Saville Kent at Road in Wiltshire in 1860 when she was fourteen. Her story has just been made into a (non-fiction) book by Kate Summerscale. The book is called 'The Suspicions of Mr Whicher or The Murder at Road Hill House'. It has just won the Samuel Johnson prize for non-fiction and is a great read. Constance originally denied the murder and there was not enough evidence to convict her but after spending some years at St Mary's and supposedly under the influence of the Mother Superior at St Mary's and of the Rev Arthur Wagner, vicar of Brighton, she confessed to the murder and was convicted in 1865. Her brother William was a biologist and naturalist and was for a time the resident biologist at the newly opened Brighton Aquarium. He lived in Upper Rock Gardens. Her case caused a great furore in Brighton at the time because Arthur Wagner refused to divulge in court what she had told him in confession. Wagner was an Anglo-Catholic and the Anglicans of Brighton were outraged at his 'popish' ways. Members of his congregation had stones thrown at them in the street and Wagner himself was fired at.
Because of her youth at the time of the crime she was given a sentence of twenty years in prison which she served in its entirety.