i found it y'all. it's called Deisui Shite Osotta
Wait the synopsis doesn't really match with the manga at all. ʘ‿ʘ I thought you were asking for a completely different one.
lol yeah sorry. deisui shite osotta was the one i was THINKING of. but i also distinctly remember where a dude does have a crush towards his sister's husband so feel free for anybody reading to help me find it (=・ω・=)
If it's an old manga ik this one. https://www.mangago.me/read-manga/ani_no_senaka/
hey it's been almost two months and idk if you're still wondering but i finally found the manga i was talking about
it's called oji shikkaku by sansaiji. and just a heads up but it's also mostly straight up porn lol
does anyone remember this one completed manga where the mc has been in love with his sister's boyfriend turned husband and his nephew, who happens to look real close to his dad (mc's crush/brother-in-law), also happens to be in love with the mc.
i forgot what happened exactly but there might've been a part where they had sex while drunk and the mc accidentaly saying his bil's name
also the mc might have been adopted but im also not sure on that