Voir Yosuga no Sora: In Solitude, Where We Are Least Alone, Season 1 - Episode 3

When Kazuha realizes that Haruka has overheard her conversatoin with Akira, she attempts to clear up any misunderstanding. She reveals that Akira is, in fact, her illegitimate older sister, born of a different mother. Due to her family's societal standing, Kazuha's father is not able to acknowledge Akira as his daughter, causing Kazuha to be resentful of her father and to feel a burden of guilt and responsibility towards the always-hard-working Akira.