Lord John and the Brotherhood of the Blade by Diana Gabaldon

In this book Lord John Grey investigates the death of his father (which happened a long time ago), looking to clear his father of charges that he had been a Jacobite sympathizer and traitor. His mother remarries, and although he gains a stepbrother who at first glance appears to be in perfect sympathy with him, he learns exactly how far he can trust his family. Again, I found the historical detail to be much more compelling than the mystery plot.