Elizabeth "Lizzie" Liddell was the eldest child of Arthur Liddell and Mrs. Liddell and the older sister of Alice Liddell.

At the age of eighteen, Lizzie was s/a and killed in the fire that also killed her parents and burned down their family home in Oxford.

The two parents are roles within the show, but I don't think I will need to cast them for the album.
The Liddell family has multiple appearances throughout the show. Lizzie will sing a lullaby to Alice on occasion, and has a solo in one of the final songs.
Her vocals require a feminine soprano. It doesn't have to be too strong, her voice is very soft.