The book for December is The Easter Parade by Richard Yates.
"Even as little girls, Sarah and Emily are very different from each
other. Emily looks up to her wiser and more stable older sister and is
jealous of her relationship with their absent father, and later her
seemingly golden marriage. The path she chooses for herself is less safe
and conventional and her love affairs never really satisfy her.
Although the bond between them endures, gradually the distance between
the two women grows, until a tragic event throws their relationship into
focus one last time."