“The family couldn’t get her out of Germany, and my mother carried this with her for her whole life,” said Katharine Wild, Max and Martha Liebermann’s great-granddaughter. “This kind of family tragedy gets passed along to the children, and I am no exception.”

The portrait of Martha Liebermann is on a Gestapo list of objects seized from her apartment after her death, according to Jutta von Falkenhausen, a lawyer who represents the Liebermann heirs. Georg Schäfer purchased it in 1955 from a Munich dealer. The Liebermann family first tried to recover it more than 10 years ago.

“I am trying to carry on what my mother and sister were doing and continue that work,” Ms. Wild says. “What I would like the people in Schweinfurt to know is: We have an opportunity. We could settle this matter.”

Two other works in the museum are being sought by the heirs of Therese Clara Kirstein, a German Jew who committed suicide in 1939 after her escape to the United States was blocked. The heirs believe the works, a drawing by Adolph Menzel and a Liebermann study, were sold under duress shortly before her death or, more likely, confiscated and sold shortly after.

“We want to have the provenance reports for those two works,” said David Rowland, a New York lawyer representing one of the Kirstein heirs, “and we would like the foundation to apply the Washington Principles. We’ve been asking for that for a long time.”

Lawsuits to recover Nazi-looted art generally fail in Germany because of statutes of limitations and other rules that favor good-faith buyers of stolen items. Claimants trying to recover stolen property are reliant on the good will of the private collectors who possess it. Some private collectors do choose to abide by the Washington Principles. One notable example is the family-owned company Dr. Oetker, a maker of baking product and food products, which has so far given back seven works to the heirs of collectors who had been persecuted by the Nazis.