But it's Dinsmoor's niece, Estella, who becomes the focus of Finn's fantasy life--and art.
Estella, Dinsmoor's neice, became Finn's muse. 
mnli hypothesis: Estella, Dinsmoor's neice, became Finn's muse.  premise: But it's Dinsmoor's niece, Estella, who becomes the focus of Finn's fantasy life--and art.