“I never told Polly how that woman clouded his judgment. No, I held that secret for him with all the others.” 

Patsy is acknowledging her role as the guardian of her father’s secrets. This comment, made while she’s still a teenager, indicates just how long she’s already borne that burden.

“It wasn’t the stare of an old man, a humiliated lover, or a widower resigned to bachelorhood. It was the stare of a man who contemplates damnation and salvation.” 

Patsy observes her father’s attraction to Sally. Her comment implies that this is something more than a simple dalliance, and the risks it carries are enormous. 

“‘And he knows you least of all. You’re his Amazon disguised as an angel.’” 

William makes this observation about Patsy’s true nature. It indicates not only Jefferson’s self-absorption, because he can’t see this trait in his daughter, but also Patsy’s inability to understand herself.