The fourth child
The fourth child in the book is symbolic of Johanna's loneliness and her desire to never be alone after her triplets leave for college. In fact, Johanna wouldn't have decided to have another child if her marriage were more secure and if her children didn't leave the house.
Wealth
Many people think that wealthy people are exceptionally happy. The Oppenheimer family, however, is far from happy. In fact, they are more miserable than most despite their wealth, defying norms and expectations.
IVF
The Oppenheimer family - like many other families who use IVF - wants to and expects to have one child. Their expectations, however, are shattered when they have three beautiful, healthy children. This leads to conflict and symbolizes difficulty and change for Johanna and her husband.
Harrison, Lewyn, and Sally
Harrison, Lewyn, and Sally are bound by blood and have spent their whole adolescent lives together, but feel no connection to each other and try their hardest to get away from each other. Most siblings - despite their differences and their family situation - try to stay connected with each other. In fact, many siblings remain friends with each other, making the triplet's situation unique and unexpected. They symbolize a weakness of family bonds in the work.
