Unanswered Questions - The Lamberts of Pacific Knoll, #6

In this final story in the Lamberts of Pacific Knoll series, parents Nathan and Iona Lambert have had a good life together for more than forty years. Only their youngest son, Logan, still lives at home.  But when he and big sister Debra are caught and injured in a landslide, Logan's true parentage is revealed, shocking the entire family. Nathan, with whom Logan has been especially close, reacts badly to the news.
But it's Iona's reaction that makes the least sense to the rest of the family. How could she not have known that Nathan wasn't Logan's birthfather? And if she did know, why has she never said anything to anyone?  Iona's subsequent health crisis forces Nathan to take stock of what he really wants in his marriage. But what does Iona want, now that everyone knows she cheated on her husband? Will their marriage survive what has come to light so unexpectedly?