There is a suggestion in the play that Chris always knew about his father's guilt.  It is hidden in the fact that Chris has never allowed his name to be put on the business, and in the remark that everyone in the town knew that Joe was guilty.  (I wish I could provide you with the exact references in the play, but I don't have a copy here.)  Chris lived with what he knew/suspected and might have spent the rest of his life in the nowhere land until Annie and George came back and Joe, in explaining what happen to George, made the mistake of stating that he had never been sick --- a direct contradiction of what he said when he passed the blame along to Steve.

At that point, it is clear to Chris, idealist that he is, that Joe is guilty and that he needs to assume responsibility for what he has done; at this point, though, we do not know that Larry killed himself because of his shame at what his father had done.  When they all read Larry's letter, Chris becomes increasingly incensed at his father.  For his part, Joe comes to realize that he is responsible for the death of his son, part of the family that he claims he has always acted for, and goes into the house to commit suicide.

At that point Chris seems to realize what "he" has done, and the play ends with Chris in Kate's arms and many viewers wondering if Chris really did the right thing ....