S1·E3 Unjoined, Unseparated

When Kazuha realizes that Haruka has overheard their conversation, she chases him down 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. To help lift Kazuha's spirits, Haruka invites her out on a date, and they subsequently realize and admit their rapidly-progressing feelings towards each other. Meanwhile, Akira continues to try her best on her own, but collapses, apparently due to exhaustion.