The irony of Jason's sketch
When Catherine draws Jason, she originally gets in trouble for it, because her actions are assumed to be malicious. Actually, her willingness to be friends with Jason is a bit ironic, especially given the frustration she feels about her brother's handicap.
The irony of Kristi hearing David's screams
The last thing Catherine wants is for Kristi to know a lot about David's autism. She wants to be normal and to have a chance for a normal friendship, but the very first words out of Kristi's mouth are, "Is he okay?" meaning that she is acutely aware of Catherine's unusual, uncomfortable situation.
The ironic relationship between Catherine and Kristi
The tension Catherine feels toward Kristi doesn't go away. Kristi represents two completely opposite, ironic aspects of society—firstly, she represents Catherine's ability to be successful with people, even though she has an unusual family life, but also, Kristi represents the unfortunate side of society, and she often comes between Catherine and Jason, and she even shows a nasty attitude toward the disabled, although it's very important to remember that Kristi is just a child, and she just got surprised, so even though her reaction is hurtful to Catherine, it's not something Catherine can just dismiss. These things are perplexing to her.
The tension between Catherine's duty to her family and her duty to herself
By being socially successful, having fun with a friend, Catherine must choose against her family and David's OT. She also chooses against Jason, too, a person who arguably needs Catherine for many of the same reasons Catherine would want Kristi's friendship.
The irony of the boy who can't speak, and voicelessness
An unexpected aspect of Catherine's character is that she literally gives a voice to another person. This ironic gift is a beautiful example of what Catherine's super powers will be later in life, and it's an indication that one day, Catherine will understand that David is a blessing to her.
