Why is Catherine more attentive to David than either of her parents? Is she just overly concerned about being embarrassed or are there other reasons behind her worries?

Why does David continually drop toys in the goldfish bowl and immediately go to tell Catherine what he has done? Is it possible to guess how David feels about his sister? When Catherine, her mom, and David drove up and saw Kristi and Ryan in the front yard, what was David feeling in comparison to what Catherine was feeling?

What could Catherine do to change the way Ryan treats David other than yelling at him and getting in trouble herself in return? Is there a point at which Catherine should involve adults?