BL reccs

This
https://www.mangago.zone/read-manga/blue_sky_complex/
The seme has an older brother and two younger siblings. Uke has non, and there's scenes where he plays and take care of the seme's brothers.

Hmm these are similar to what you described -

Lots of young siblings x no siblings but shounen ai - https://www.mangago.zone/read-manga/kanemochi_kun_to_binbou_kun/

lots of siblings x one sibling but pretty lonely at home - https://www.mangago.zone/read-manga/itsuka_tomodachi_ja_nakunaru_to_shite_mo/

mostly siblings of the same age but one cute younger brother - https://www.mangago.zone/read-manga/tenjou_no_shita_ni_koi/

A pair of brothers and their stories: https://www.mangago.zone/read-manga/sannin_gurashi/

Only the first chapter matches your request, because then the rest of the chapters deal with one of the younger siblings, but he’s the bottom:
https://www.mangago.zone/read-manga/sweet_pain/