One Moment Early In the Film That Muddles the Plot...

It is kind of foreshadowing, though. It is to show that Corrine and Cathy do not have the smoothest relationship (hence later: "Always, it's you!" then slap...and slap) That moment later does not come as a surprise because right from the beginning (this particular scene followed sporadically by a few others) it is demonstrated that Corrine has always had issues with Cathy. Cathy and her father were closer. The voice over in that scene says how "he never let me forget that I was his favorite." Simultaneously, Corrine was always closer with Christopher. So, if she were to watch Chris Sr. and Jr. together, she would probably be beaming with joy. However, this scene with that little moment is to have many layers demonstrating to the audience that something is a little off. It shows that (and is explained later) that Corrine has a little bit of a dark side...and in particular, a resentment against her daughter, Cathy. The audience is supposed to wonder for a moment what it is about in that moment to put them slightly on guard. Yet, it dissolves into a (brief) happier moment in order to have the audience think back on it later. It becomes an opportunity to suggest that because Cathy is closer to her father, there is already a dividing wall between her and her mother. At this time in their lives, it is not crucial but it definitely contributes to what happen later on. There are lots of little moments in the film where Corrine is looking off into space or focused on something that makes the audience wonder what exactly is she thinking. She isn't as great as she pretends to be.