Searching the web and my library it seems that little is known about Fra Filippo Lippi's second child by erstwhile nun Lucretia Buti, - Alessandra Lippi, - but this story uses what information there is and makes a reasonable proposition that since her mother was a painter's model it is reasonable to propose that her daughter would take over the role of greatest beauty in the fullness of time.
What I have found difficult since reading this book is knowing whether paintings that I had previously been taught portrayed Simonetta Vespucci, are indeed her - or - as suggested by Carolyn Street La Fond - actually Alessandra Lippi? Given the disparity of social position between artist's models and noblemen's wives there is some credibility that it would have been Alessandra who posed for Botticelli - and it is nice to think that she survived infancy - but who knows the truth now?
This book is an easy read certainly and contains some nice provable historical details along with the fantasy!