Why does Frau Holtzapfel suddenly stop going to the bomb shelter during air raid drills?

A few weeks after Hans breaks his leg, Michael Holtzapfel returns home to tell his mother, Frau Holtzapftel, that his brother, the Frau's other son, has died at Stalingrad. Frau Holtzapfel becomes so deeply depressed that she doesn't care if she lives or dies, and she refuses to enter the bomb shelter during air raid drills.