There are any number of middle grade books involving Sir Arthur Conan Doyle's immortal character of Sherlock Holmes, including Andy Lane's fantastic Young Sherlock Holmes, Hearn's Baker Street Academy, Cavallaro's Charlotte Holmes novels, Misri's Jewel of the Thames and Springer's Enola Holmes series, and the Lock and Key series is a nice twist, since the main characters are the Moriarty family. Moira is leery of her family's past, but James seems more willing to embrace it, and the exchanges they have as they work through their family legacy is interesting.
There are few murder mysteries for young readers, and they are much in demand. The killing of the lawyer was an appropriate choice-- close enough for the children to be concerned and affected, but not a family friend to add to their already considerable grief. Tying in the most recent murder to the long history of family tragedies wrapped up many of the plot lines nicely as well.