Ai Hoshina's nipples from her cute little sister's floating bra have become an erotic relationship even though she is a porori brother and sister

The movie explores the development of an erotic relationship between a brother and sister, Ai Hoshina and her 'porori brother'. The title suggests that their relationship becomes intimate despite being siblings. The central dramatic tension likely revolves around the taboo nature of their relationship and how they navigate these forbidden feelings.