From Mental Health Law Online
(1) The operation to separate Siamese twins, both of whom would otherwise shortly die, would clearly be in Jodie's best interests, but would kill Mary; the court must perform a balancing exercise in determining what was in the twins' best interests, regardless of the parents' religious objections; the scales come down heavily in Jodie's favour. (2) The operation would be lawful in criminal law, under the doctrine of necessity, and as the primary purpose was not to kill Mary
Without separation both Jodie and Mary would shortly die. Mary relied on Jodie so would die on separation. The parents objected to the operation because it would kill Mary. The court needed to determine what was in the best interests of the twins (family law) and if the operation would be lawful (criminal law).
The court held that performing the operation would be the lesser of two evils and should be permitted. The best interests of the twins was paramount rather than the interests of the parents. The operation would be lawful under criminal law under the doctrine of necessity, and as the primary purpose was not to kill Mary.