In the book, Devra's little sister is Deaf/hard-of-hearing, and her counterpart (Kai Locke) is mute/hearing. Devra is fluent in BSL. Kai Locke is fluent in ASL, but his BSL is OK. Devra eventually learns a bit of ASL just observing Locke and Locke's associates. The both of them eventually mix the two languages together--adopting the easier one-handed finger-spelling of ASL, but still using British signs for the most part.
@Blondemaverick, interesting! Thanks for explaining, I was curious, being Deaf and fluent in ASL myself. If you ever have questions with ASL or Deafness, ask away!