It was really grimish and somewhat incesty-like. I remember reading it in the 2000s but forgot about it. I'm interested in rereading it but can't remember the name

Oneshota implies that the sister is the older one and has a little brother. So what you're saying doesn't add up unless there are more than two siblings.

Also could you let me know more details or even better if you found out the name of the manga?

What genre is it anyway? Seinen, hentai, doujinshi, oneshot, ecchi, slice of life, etc.

the title is a typographical error, it was meant to say oneshot,

other details, is like. the brother is actually older. the oneshot is grim and uncomfortable. there's monologues of the brother's mourning i think. the manga ends with the brother "kissing" his dead sister. it was like mid-2000s i think.

Could it be Sono Saki no Fuukei? It's not a oneshot but it's very short, and it's grim and incestuous.

Doesn't seem like it, I'm looking for the brother attending his sister's funeral and one of the final panels is him kissing her. Only to imply she's not actually dead.