Are you researching or the cousin-romance trope?
If you want emotional tension between a brother-sister duo that feels romantic without crossing the line, use this structure:
(sister) with a prefix denoting the specific aunt or uncle, such as (mother’s sister’s daughter). Popular Tropes
Example: A male lead pretends to be a “brother” to get close to a girl, but his intentions are romantic. Outcome: Almost always condemned in mainstream media as daga (betrayal) and beizzati (dishonor), leading to the male’s downfall or repentance.