"The Sinclairs are athletic, tall, and handsome. We are old-money Democrats. Our smiles are wide, our chins square, and our tennis serves aggressive." 

Cady's family has very distinct traits that are nicely summarized in this passage. They are very wealthy, and wealth provides them a sense of power, which is embodied in their square chins. They feel they must pretend things are going well even when they aren't—hence, their unwavering smiles. But they are not always nice. They can be overly aggressive. Further, tennis is historically a favored sport of the white, wealthy upper class.

“Then he pulled a handgun and shot me in the chest.”

One of the more interesting sentences in the novel, this phrasing depicts Cady's suffering over the loss of her father through the metaphor of murder. She perceives his departure as harming her in the same way a gunshot might. This sentence may also be viewed as foreshadowing for the accidental murders of the Liars that follow.

“He married Tipper and kept her in the kitchen and the garden. He put her on display in pearls and on sailboats. She seemed to enjoy it.” 

The gender relations in Cady's grandparents' world are very different from the more egalitarian ones she grow up with. Her grandmother, Tipper, is a "trophy wife," a beautiful woman a wealthy man marries and shows off to the world. She enjoys being put on display, and in the descriptions in these sentences, she’s rendered virtually inanimate.