The Forgotten Affair of Youth (Isabel Dalhousie #8) by Alexander McCall Smith

Happy with her husband-to-be and beloved son, Isabel Dalhousie has feelings about parenthood that grow more tender daily. So when Jane, a visiting academic adopted and sent to Australia as a baby, asks for help in tracing her Scottish origins, she cannot refuse.

However, habitually upright Isabel finds herself beset by temptation - for instance, to be suspicious of Professor Lettuce's latest subterfuge, and of her niece Cat's weakness for the wrong man. And when the search for Jane's parents turns troubling, she can hardly prevent herself from interfering too forcefully in family secrets. As she steers a course between love and laissez-faire, our philosopher heroine succeeds in resisting all temptations but those which must be answered, and teases a solution from every problem.