Rosanna and her children fled to Venice where she and her eldest son, Francesco, used Perception filters to appear human. They had come to Venice because of their marine nature and to exploit the fear of the plague. They set up a "school" under her management. It gained fame in the city as a plague-free haven and became fashionable.
Once the girls were in the schools, their blood would be replaced with that of the Saturnyns, and they would be transformed into females of the species, ready to help drown Venice and form a new race with the other male survivors. The Eleventh Doctor disabled her machinery, leaving her plans in tatters. With no hope of rebuilding her species and her perception filter broken, Rosanna was stuck in human form. She threw herself into the canal and was devoured by her children because they could not recognise her and thought she was a meal. (TV: The Vampires of Venice)