The Doctor's Daughter by Sally Quilford
We're back in 1916 for this one - the first Peg Bradbourne mystery. A man is found dead, apparently suicide but there are some unexplained circumstances surrounding his death. Then suddenly Peg's own stepmother is found dead, and Peg finds herself promising her little half-sister that she will get to the truth. Are the deaths linked and if so, how?
The plot progresses with many twists and turns, typical for a Sally Quilford novel, and when the truth comes out it's not at all what you expect. The perfect novella to while away a few hours.