Hey I'm not looking for any manga in particular but do you happen to know any yaoi/shonen ...

Hi, this is not exactly what you're looking for but this is the closest I could remember reading :)
The protagonist falls for his sister's boyfriend (and not brother) : http://www.mangago.zone/read-manga/i_want_to_become_your_bird/

http://www.mangago.zone/read-manga/mimurake_no_musuko/

http://www.mangago.zone/read-manga/i_want_to_become_your_bird/
this second one is the sisters boyfriend, but definitely worth the read!

http://www.mangago.zone/read-manga/love_songs/

have fun! ヾ(❀╹◡╹)ﾉ~

Thank you!! I also like this kind of plot ლ(´ڡ`ლ)

Thanks a loooot