I don't think these fit what you are looking for exactly but they are about older siblings that have to deal with raising young siblings (related and unrelated) that get left behind after their parents deaths:
Note - these both have inseki (like incest but the couple is not related by blood, they're step-siblings)
https://www.mangago.me/read-manga/super_lovers/
https://www.mangago.me/read-manga/bokura_no_negai/
There's also this one (BL but no inseki):
https://www.mangago.me/read-manga/sado_no_otoko_o_otosu_houhou/
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Additionally this one is shounen and a classic. The older brother doesn't raise the younger one in this case. They're a team working together to reach their goals:
https://www.mangago.me/read-manga/fullmetal_alchemist/
Thank youuuuu, it doesn’t matter as long as there is an oldest sibling and even better if they’re struggling (๑•ㅂ•)و✧
Someone pls recommend me something about oldest child struggling and then got their happy ending, it doesn’t matter if it’s bl, gl or straight