When Eliezer and the other Jewish prisoners arrive at the camp, the SS men constantly bark instructions at them. The most important of these is "Men to the left! Women to the right!" Inmates of the Nazi death camps were segregated according to gender: women and girls would be housed in one block, men and boys in the other. While men and boys were put to work, women and girls tended to regarded by the Nazis as more expendable. Upon separation, many of them were immediately sent to the gas chambers to be killed.
This is what appears to have happened to Eliezer's mother and sister not long after they were separated from him and his father. The Nazis cynically sent girls to their deaths alongside their mothers, as they thought this would make the process of killing them a lot easier—easier for the Nazis, that is—by minimizing potential outbreaks of hysteria. They had no moral qualms about the actual process of gassing innocent women and children to death; they simply wanted to make it more efficient.