I feel like I've read something with a similar plot before, but I don't remember the title...

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

or this one:
http://www.mangago.zone/read-manga/kuro_koi/

Thanks for the links! Unfortunately, the series I was thinking of wasn't about twins. I'm unsure if I'm recalling all the details correctly, but I think the younger brother was in middle or high school, while the older brother was a working adult? Sorry for being vague haha my memory isn't that great

I'm thinking about Junjou Romantica. It's reverse tho, younger brother standing in for the older one, but with the same text "why him, anyone will do" ლ(´ڡ`ლ)

http://www.mangago.zone/read-manga/blt_burning_love_twin/. Maybe this one?

sounds interesting but unfortunately i haven't read anything like that. good luck~

It's a bit similar to this:
http://www.mangago.zone/read-manga/castle_mango/

This one no ...?
http://www.mangago.zone/read-manga/kitto_kiss_o_suru_toki/

YES IT WAS THIS ONE THANK YOU

And thanks to everyone else for your help as well! ٩(๑❛ᴗ❛๑)۶