I think, that Nico was already dead when she spoke with the big brother in front of the house. That's why she dissapeared suddenly. He think she went back into the house but nope. Since Bansai was just behind and took him by the arm. Surely he would have seen his sister run back into the house.
So basically, she was already dead, her elder brother, must have seen her corpse when he saved the kid, that's why he told the other one to give up.