Sorry if can't give the best clues. I'm looking for a series I saw here a while back and can't remember the name. To the best of my knowledge, I remember the synopsis saying it was about a sister and a brother who fell in love and had to keep it a secret from their parents. I can't remember if the two were twins or if they were just really close in age.

I know for a fact it wasn't a shonen manga, but I can't remember if it was shojo or seinen. I think the cover on its series page had the two laying down near each other and the background was a cityscape, or space, or maybe just really dark.

Sorry again if my clues aren't very good. Thanks anyway.

Boku wa Imouto ni Koi wo Suru?

that might be it, but I swear I seeing another one sort of like that.

flowers of evil?

Hitsuji no Uta?