From day one, Wendy has always adored Louie, her adorable baby brother. In fact, she's always been there to take care of him and help her parents out, and she never resents him getting more attention than her. She basks in the role of big sister:
Lost without her. Her mom and Josh, wandering in a forest like Hansel and Gretel, but with Louie in the front-pack. Calling out, Wendy, Wendy, where are you? We're lost without you.
Of all the things I ever got to do, she told her mother on the way home, being a big sister is my favorite. (9.30-31)
Louie is the baby of the family and can always count on his parents and big sister to take care of him and coddle him. He's felt safe and secure his whole life, which is why he takes it so hard when 9/11 happens and his mother doesn't come home. Nothing in Louie's life has ever prepared him for something like this.
Wendy and Josh may let out their grief through tears and words, but Louie is just a little kid, so he has a much different way of coping with his mother's death. Because Louie doesn't really understand that his mother is gone, he keeps hoping she'll come back—and when she doesn't, he experiences all of that grief and loss anew. He acts fine and wants to play a lot of the time, but then he'll start throwing a huge tantrum out of nowhere:
Before, Louie had only struggled a little. Now he was thrashing in all directions. He tore the page out of the coloring book and ripped it in bits that he threw at Josh. He kicked so hard his shoes flew off. I hate you, he said. Get out of here. I wish you'd fall out of a building. (5.23)
Louie's anger and grief comes on suddenly and without warning, which makes sense because Louie is still so confused about why his mother is gone. He can't understand it yet, and it's going to take a lot more time before he can grasp that she's truly dead. When Wendy leaves for California, Louie loses another beloved family member, which only makes us extra glad that she decides to return to New York—Louie needs her.