Mavis was the name of the real Altounyan child who Titty was modelled on, NOT of the character "Titty" in the books.
"Mavis" was also the name of the real boat that the "Amazon" was named after. Arthur Ransome and Ernest Altounyan bought the boats Mavis and Swallow in order to teach the (real)children to sail.
The Altounyan children were:
Taqui (thought to be the basis for Nancy Blackett)
Brigit (who appears as "the ship's baby"
"John" was added for balance as Roger was the only male Altounyan child.
Christina Hardyment wrote probably THE definitive book linking real people and places with Ransome's stories. It's called "Arthur Ransome and Captain Flint's Trunk", written about 10-12 years ago and well worth a read for anyone with an interest.