those hearts at the end, I think it was her the one that is a real brocon, and she always threw him insults about him gazing her as she got too nervous from him looking at her.
Bro has show genuine cafe for them to be a family, he took the advice of the friend but not in the romance sense but in taking care of her and remind her that she's loved.
She was hated by her girl classmates and lusted by the boys, her self confidence is paper thin, so just one person saying to her in the past that her brother wasn't a real one and that lusted over her and her perception of him and the family that cared about her broke. (she believed easily because she's kind of a moron 😅)
... just my two cents, it could be sth like this or probably nothing as it was made mostly from hentai doujinshi into a serious one hahaha.
Big sis never underestimates the horny power of lil bro. She's seen the doujinshis
most of the dj are from other people, like the bad choice taken in this one lead to her getting... yk.