Shopaholic & Sister by Sophie Kinsella

Becky Brandon and her husband, Luke, have been on a extended round-the-world honeymoon when they decide to return to London. Upon coming home, Becky finds out that she has a long-lost sister, Jess. Becky and Jess are completely different people. Becky is into shopping and the finer things in life, while Jess loves saving money and hiking to find rocks. Will Becky be heartbroken when she and her sister don't see eye to eye? Shopaholic & Sister is the fourth book in the popular chick lit series. Sophie Kinsella is the pseudonym of Madeleine Wickham. I've read and enjoyed several of those books (The Tennis Party, Swimming Pool Sunday, and the Gatecrasher) and they are different from the Kinsella books in that they are concerned with deeper family issues and are not humorous.