We only see Ethan's Aunt Caroline in 11.27, the chapter where Ethan invites Lena over for Thanksgiving. She's a mirror image of Ethan's mom, except for how they dress. Her sister, Lila, loved wearing jeans and going barefoot, but Caroline is fonder of sundresses and sweaters.
Caroline is an expert in architecture and antiques. Ethan says that Caroline "liked to see the expression on people's faces when they found out she was curator of the Savannah History Museum and not some aging debutante" (11.27.39). We like people who defy expectations. This book is full of them.