Complex sentence: The novel's explanation for their separation was Katherine's shock over revelations concerning the death of her husband, but they eventually reconcile and marry after the death of the Duke's second wife, and after their children are grown.
Split into simpler sentences: The novel's explanation for their separation is Katherine's shock over revelations concerning the death of her husband. However, the couple eventually reconcile and marry after the death of the Duke's second wife and after their children are grown.