Tiny little thing / Beatriz Williams.

Enhanced descriptions from Syndetics:

In the summer of 1966, Christina Hardcastle--"Tiny" to her illustrious family--stands on the brink of a breathtaking future. Of the three Schuyler sisters, she's the one raised to marry a man destined for leadership, and with her elegance and impeccable style, she presents a perfect camera-ready image in the dawning age of television politics. Together she and her husband, Frank, make the ultimate power couple: intelligent, rich, and impossibly attractive. It seems nothing can stop Frank from rising to national office, and he's got his sights set on a senate seat in November.
 
But as the season gets underway at the family estate on Cape Cod, three unwelcome visitors appear in Tiny's perfect life: her volatile sister Pepper, an envelope containing incriminating photograph, and the intimidating figure of Frank's cousin Vietnam-war hero Caspian, who knows more about Tiny's rich inner life than anyone else. As she struggles to maintain the glossy fa#65533;ade on which the Hardcastle family's ambitions are built, Tiny begins to suspect that Frank is hiding a reckless entanglement of his own…one that may unravel both her own ordered life and her husband's promising career.