Haruki hears about Kazusa's moving from her mother, who she hadn't been on good terms with.  He feels betrayed and angrily confronts her.  But his words force open the door to Kazusa's heart, which she had desperately tried to keep closed so that the three of them could stay together.
How can you say a line like 
that with a straight face?
Wie kannst du das 
mit so einem ehrlichen Gesicht sagen?
¿Cómo puedes decir algo 
así sin echarte a reír?
¿Cómo puedes decir algo 
así sin echarte a reír?
Como consegue falar essas
 coisas de cara séria?