Jane Fisher was Phryne's sister who disappeared. The disappearance haunted Phryne for years.
Jane was born September 21st. Since her father was drunk when he registered her birth, he wrote Phryne's birth date, December 21st, on Jane's certificate. That mistake was what got her killed. Murdoch Foyle was searching for four girls who shared his birth date (December 21st) to be goddesses that would protect him in his journey to the afterlife. Through an old student who worked at the office for birth, marriage, and death, he got access to the birth certificates and selected his four victims from the neighborhood of the university at which he worked. During a circus performance that Jane and Phryne were watching, Jane vanished. Her body was recovered years later and buried in the family plot.