What do people mean when they speak of “family loyalty”? How does Kiyo express his loyalty when he defends his mother? When he apologizes to his father?

When Kiyo steps in to protect his mother from his father's threats he defends her. Kiyo understands what his mother has endured while his father has isolated himself. His mother has faced the rumors wbout her husband's allegiance, while his father has hidden from them. Kiyo's apology to his father comes from his feelings of loyalty and is given out of respect for his father's place in the family.