What is the turning point of the plot in "O Pioneers" by Cather?

I would argue that the turning-point in the story is the murder of Marie Shabata and her young lover, Emil Bergson. Marie is an unhappily married woman who's been involved in an unconsummated love affair with Emil, the brother of Alexandra, the story's protagonist. One evening, Frank Shabata, Marie's husband, catches the two love-birds together in the orchard and, in a fit of drunken, jealous rage, shoots both of them dead. This is the turning-point, because it is the highest point of tension and drama in the story, whose resolution prepares us for the falling action and denouement.
The character who undergoes the most radical change in the wake of the double murder is Alexandra. She's worked tirelessly over the years to make a go of her late father's farm. She's tried so hard to make a life for herself and her family on the prairies at a time when so many other families have left to seek opportunities elsewhere. And yet, in the wake of her brother's tragic death, it appears that all that hard work, all that sweat and effort, and all those sacrifices, have ultimately been for nothing.