Triple Jeopardy by Anne Perry
In the second novel of this series, Perry introduces Jemima, Daniel's sister, and her husband, Patrick, whom Daniel has never met, and their daughters, Cassie three and Sophie one. They have been living in America, Washington DC to be exact, and Daniel, his mother and father lives in London.
Again it's Daniel who has the education to be a lawyer, Marcus Croft and his daughter Mariam, Kitteridge, Roman Blackwell, Impney the clerk, and the whole host of characters from “Twenty-One Days”, her first novel in this series.
Philip Stanley is a British diplomat stantioned in D.C. When he was supposed responsible for breaking and entering a house in D.C. and stealing a necklace from Jemima's friend Rebecca Thorwood. But claims diplomatic immunity and hightails it back to England. Sidney winds on trial for embazzement. Thats when Daniel Pitt gets involved. He is hired to defend Sidney.
Remember this is before today's technology. They had to travel and wire back and forth to get clues. This was before telephones had trans-atlantic lines and cars were new. What he finds out is totally unexpected. It has to do with the house Rebecca's aunt left her when she died. That's all the clue you're getting. Read it for a good old-fashioned mystery.