What happened to Berel's family during the attack on Poland? Did they escape to safety?

In Part III, we learn that Berel Jastrow's wife and daughter have died, and that Berel is living in a cramped space in Russia, having fled there with what remains of his family. Berel's son wants to flee from the Germans, who are moving their way, but Berel opts to wait on the chance that the Russians will repel the attack.