PLEASE DON'T MAKE THIS YAOI/SHOUNEN AI. PLEASE.
Edit in case someone misunderstands: I am fine with incest (ngl I like some incest mangas too), though I really just find their sibling relationship to be very cute and wholesome and I don't want it to be set aside just for romance. It's a common trope in comics where being very close to someone (friendship, siblings, whatever) just gets interpreted into a romantic relationship because it's "more special", ending up belittling other types of relationship.
Sakazuki-san Chi no Gikyoudai