Thy Brother's Wife (The Passover trilogy book 1) by Andrew M. Greeley

The direction of Paul and Sean Cronin's lives was shaped the day their father, a self-made multimillionaire, decided that one of his boys would grow up to be a cardinal while the other would become president of the United States.

For his elder son, Paul, the father had even chosen a wife―the beautiful Nora, who had come to the Cronin home as an orphan child years before. Obediently, and with a genuine vocation, the younger son, Sean, went into the priesthood. With a more cynical view, Paul went to Notre Dame to prepare for a life in politics until the Korean War intervened. Then came the news―Paul Cronin was missing in action.

"If he dies," Sean's father told him, "you must leave the seminary and marry Nora." The words sang in Sean's head. Could he renounce his sacred calling―and marry the girl he had always loved?

Long out of print, Thy Brother's Wife is a classic tale by one of America's most loved storytellers.