ff
I can think of several that applies to. Any more detail?
maybe just link all of them ~~
The older sister tells her younger brother about sex between men and allows him to read her manga. She later finds out that her brother is the bottom, while anyone who knew about them would assume the opposite. Mainly because of their personalities.
(=・ω・=)
http://www.mangago.zone/read-manga/katekyo/
http://www.mangago.zone/read-manga/hanayome_kun/
http://www.mangago.zone/read-manga/hanamuko_san/
http://www.mangago.zone/read-manga/u_don_t_know_me/
http://www.mangago.zone/read-manga/kazoku_ni_narouyo/
http://www.mangago.zone/read-manga/sayonara_game_minaduki_yuu/ and the sequel
http://www.mangago.zone/read-manga/itsuka_tomodachi_ja_nakunaru_to_shite_mo/
http://www.mangago.zone/read-manga/suzume_favorite/
http://www.mangago.zone/read-manga/renchin/ but first read first bite then renchin
http://www.mangago.zone/read-manga/jackass_sawatte_ii_tte_dare_ga_itta_yo/
There is a yaoi where the sister knows they are dating and is a total fujoshi. I cannot for the life of me remember which one it is
┗( T﹏T )┛
Also recommendations for yaoi. I would like to read more where someone in the family is aware that they are gay/bi; or where it is more centered on telling their family(s).