The play reaches its turning point at the end of act two. At this point the outcome of the action is determined. What is the turning point of the play ?

Keller claimed that he had the flu and could not go into work. This excuse made it possible for him later to deny any involvement in the shipment. Chris says he heard all this before in court, but George says it was different hearing it directly from his father, a "frightened mouse" of a man who would never do such a thing on his own volition. Chris counters that he certainly would, and because he was such a frightened mouse he would throw the blame on someone else because he was not man enough to take the heat. George accuses Chris, saying that he must know the family secret, and that this is why his name is not on the business. Chris warns him not to start a fight.

http://www.gradesaver.com/all-my-sons/study-guide/section2/