Original Sin (a Peter Brichter mystery) by Mary Monica Pulver
'Kori Price Brichter planned on a wonderful Christmas. She and her police officer husband, Peter Brichter, had invited their friends, Frank and Mary Ryder and Gordon Ramsey, to spend the holiday with them, their son Jeep, and their nanny, Jill. Most important, Kori had invited Evelyn Biggins to join them. Having believed that all of her relatives had died, Kori was particularly thrilled to have found Evelyn, now her only living relative, to tell her of her family history. But Evelyn had only been in the house for a few hours when she was found, dead, in the house's library, victim of an apparent murder. Since the guests were now isolated in house because of a blizzard, the only possible conclusion is that one of them had killed Evelyn. But who? '