Familial bond
Though Harrison, Lewyn, and Sally were born to the same parents on the same day and within minutes of each other, they do not feel a strong "familial bond" with their siblings. This creates the central conflict of the book: the siblings work hard to get away from their siblings and are crestfallen after the birth of a new sibling, who they must also deal with.
Relationships
Harrison, Lewyn, and Sally's parents are growing more distant from each other and becoming more desperate. They are, in other words, not in a good place to have another child. However, despite this, they decide to have another child as an example of irony in the work.
Walden School of Brooklyn Heights
The Walden School had a "socialist ethos" yet "stood in bald contrast to soaring [and exorbitant] tuition." Socialists would expect school to be free for everyone; charging for such a service (especially charging such high rates) is unexpected and ironic for self-proclaimed socialists.
The kids
Each of the three kids - Harrison, Lewyn, and Sally - lives with tremendous privilege and opportunity that few others would have. Ironically, they are not happy with their lives and want desperately for them to change. This illustrates how out-of-touch the kids are and is a searing indictment of their parents, who raised them in New York and Massachusetts.
