That way murder lies : a Mitchell and Markby mystery / Ann Granger.

Enhanced descriptions from Syndetics:

Meredith Mitchell is delighted when an old friend from her consular days, Toby Smythe, turns up on leave between foreign postings. But Toby has a problem---or rather his relative Alison Jenner has---and Toby wants to enlist the help of Meredith's fiancé, Detective Superintendent Alan Markby. Alison has been receiving anonymous hate mail in which reference is made to a time twenty-five years earlier when she stood trial for the murder of her aunt, Freda Kemp, but was acquitted. Who is the writer, and how does he or she know about this secret in Alison's past?

Markby is reluctant to become involved, not least because Toby is hardly his favorite person. Besides, he and Meredith are planning their wedding, and distractions aren't welcome. But inquiries into a poisoned pen campaign soon turn into a murder investigation.

With the help of Inspector Jessica Campbell, a new member of Markby's team, and the non-professional but enthusiastic assistance of Meredith and Toby, the inquiry unravels a twenty-five-year-old mystery and its dreadful legacy of violence.