I hope that this won't be too hard on you guys...

The manga was about a desease or something malign that was spreading trough the vilage and killing people, and there is this "clan" that is investigating what is causing it. It was in a samurai setting of sorts...
I think it had something to do with a demon possessing someone, because on the first chapters we discover that the older brother of the protagonist is the one causing it or heavily aflicted by it, but the spreading wasn't intentional. He had the demon/desease and was supressing it on his body trough some tattos placed on his neck, wrists and ankles. He was part of the "clan", had the "thing" for some a while and was trying to find a cure/resolution for it.
I remember the "clan" discovering the brother, him dying, and things going downhill for the protagonist, that didn't know about his brother situation...

This is the most details that I can remember, hope it help you guys to find it...

Edit: Found it - Donten ni Warau
It seems I was wrong about the plot... The only thing I got right was the phisical description of the brother... 😐

Story sounds similar to Kyonshii Keikaku but it’s Shonen-ai.

Thanks for the help!!
But it isn't it... 😢
I don't remember if the story was shonen-ai, but I remember it wasn't an oneshot.
But thank you, anyway!