When Karen Frostig made her way to the podium of a packed United Nations General Assembly on Friday, the projections of grainy passport photos of her grandparents, Moses and Beile Samuely Frostig, towered above her. These images had been in the living room of her childhood home in Waltham, Mass., outside Boston, the only portraits that hung in the house. But they weren’t much discussed. No one in the Frostig home wanted to talk about the Holocaust, about how Moses and Beile, Jews who lived in Vienna, had been loaded onto trains that the Germans sent into the forests of...