What happens to Vladek's sister Fela? Why does Valdek's father join her?

The Jews are all corralled in the stadium. Vladek's father is in the line of Mordecai. He is separated and spared. He tragically leaves behind Fela and her children. They are destined to go to Auschwitz. Vladek's father can't bear to leave them so he crosses over to their line. Obviously his life was not worth living if he had to sacrifice the people he loved.