Looking for this one tragedy, psychological manga.

It was about a brother and sister traveling across a plagued or diseased land, at a certain point a homeless speechless man follows them until I think the boy kills him. Then they come across a bus full of people and they slowly die one by one due to the disease. Then they leave the bus and are captured by this one guy with military I think, and then the boy works for him while keeping the sister safe. Then they escape there, some other stuff happens and the whole point is to reach the other side of the world, in a forest they get shot at and the sister dies, the sister was shot by another brother and sister who I think get shot by the brother. Then the boy grows old and dies by the sea.

Any help is appreciated I remember it being fascinating and incredibly good