Who is Eliza Mellon Swain from Slapstick: Or, Lonesome No More! and what is their importance?

Eliza Mellon Swain is Wilbur's twin sister. After their testing, Eliza is sent to live in a mental institution, while Wilbur is sent to a private school. She is later rewarded for their joint writings (as children) with a trip to Mars, where she dies in an accident.