https://www.mangago.zone/read-manga/renai_nante_yurushimasen/ (the younger brother is under a different title)
Banana Scandal
Therapy Game
I have no idea of this is what you're looking for, but I immediately thought of this. Hope you like it!
https://www.mangago.zone/read-manga/brother_complex_onii_san_kocchi_muite/
BL motel
https://www.mangago.zone/read-manga/bl_motel/
second couple is a younger brother.
https://www.mangago.zone/read-manga/iyayoiyayo_mo_kiss_no_uchi/
........
Younger brother story and ~
https://www.mangago.zone/read-manga/gosan_no_heart/
+
Older Brother Story sequel
https://www.mangago.zone/read-manga/owaranai_fukou_ni_tsuite_no_hanashi/
Any yaoi where there's a second couple which is the older or younger brother of someone in the first couple (like love shuttle for example)