The MC is shocked because his sister said something nasty about him not having kids while he thinks she knew she was pregnant - basically lording it over him despite the fact he really wants a child. The daughter cares because they've manipulated a rift between the siblings by telling the sister that Hiroto wasn't as hurt by her comments as he was, so she would announce her pregnancy and upset him. The siblings never actually talked after their fight so nothing was really resolved between them directly. The sister's child is from the affair she had.