I have a rule that contends
Uncle Joe and Aunt Mabel were not the models for the characters in the novel you are reading.
My experience with the habits of general readers, even avid general readers, is that they have a tendency to latch onto something familiar, like a character who reminds them of Sister Kate, and then to make the mistake of reading the narrative as if it was actually about Sister Kate, even if subconsciously. The reader’s discussion of the book then tends to slide into anecdotes about Sister Kate and the actual text is shoved aside.
The same thing holds true for the setting: that house might remind the reader of Sister Kate’s house on Martha’s Vineyard, but it’s not Sister Kate’s house.