The Thornthwaite Inheritance by Gareth P. Jones
Ovid and Lorelli Thornthwaite have been trying to kill each other for so long that neither twin can remember which act of attempted murder came first. But whoever struck first, trying to take each other's lives is simply what they do. Until one day a lawyer arrives at their house to take stock of its contents, and his accompanying son attracts their attention.
Soon a new battle evolves - one in which the twins have to work together to solve the mystery of their parents' deaths. Can Lorelli and Ovid overcome their old animosities, and will they ever get to finish that game of chess?