Eliza (or Beth as Phryne knew her before) is Phryne’s younger sister. She’s four or five years younger than Phryne and first appears in The Castlemaine Murders.
She’s described as being four inches taller than Phryne, large-boned and blonde, with bright blue eyes. She takes after their ‘big, florid father’ rather than Phryne who takes after their ‘thin, dark mother’. Later in the book Phryne mentions they also have a younger brother Thos who goes to Eton. A previous book had Phryne referring to her mother andsisters plural back in England, but this one only mentions Eliza and Thos. But that could be explained away in context if their other sister is already married. Anyway, I totally love that Phryne has more siblings in the books, including a younger sister who died of diphtheria in Collingwood one winter, presumably the inspiration for the whole Janey arc in the show.