In chapter 16, the commandant's car enters the camp. Reuven, a little boy, is too startled to run away and hide. Hannah calls out for him to move before the commandant sees him, but Reuven does not respond. The commandant exits his car and speaks to Reuven. He talks kindly to the boy and even wraps his white handkerchief around Reuven's bloodied knee.
When the commandant asks Reuven where his mother is, Reuven doesn't respond. Instead Hannah reveals herself and tells the commandant that Reuven's mother "died years ago, when he was born." The commandant then asks Hannah if she is Reuven's sister. Hannah shakes her head and the commandant says, "That is good. For you." The commandant then takes the boy to his car and drives away.
Reuven does not return to the camp later that day. Looking at the black "smoke curling up" from the gas chambers in the distance, Hannah realizes that Reuven has been "chosen" as another victim of those chambers. She says that his "choosing" is her fault and that she "should have said he was [her] brother."
If Hannah had said that Reuven was her brother to the commandant, Rivka tells her, he would probably have taken her, too. Perhaps Hannah feels, however, that at least that way she might have been able to comfort Reuven in his last moments. Perhaps Hannah also feels that she could have done more to get Reuven to move out of the way before the commandant saw him. In reality, there is of course little that Hannah could have done to save Reuven.