So I read this manga years ago, back around 2009/10?
The main character, a girl who's a student gets whisked away by something, from what I remember it's a river of time. She's in the future(?) and she meets some more people, and it's eventually revealed they're immortal, as they don't age anymore. However, they will die if the river of time touches them, as a small child is killed because she couldn't escape the flood, while her brother watches her. There's like gang wars too, as people fight to survive.
I believe the main character's little brother is looking for his sister, since she just up and disappeared?
It also, I believe, deals with a lot of black butterflies?
I want to say I read it on manareader.net
The translations were good. The MC I believe had medium/long black hair. She wore the typical Japanese high school outfit, the white blouse and black skirt if I recall correctly.
I want to say her getting whisked away was a nationwide event, but I can't recall. The place she lands in was an abandoned city, all except for the tribes. The groups she met at first weren't friendly, just explained the situation. The boy and his sister had been alive for hundreds of years, and were rude. They were adults trapped in child bodies. The time the boy did show compassion, was when he mourned his sisters death. Otherwise they were scornful, saying the girl wouldn't last long.