Because the fruits of the scheme rested in the sons, and if the daughter gets a divorce, the money will escape from the victims of the fraudulent scheme, so whatever she is awarded in the divorce will be out of the reach of the scheme victims. Probably billions involved, and that's how Madoffs planned it. The scheme victims should intervene in the divorce case before its too late. She filed the divorce the day Madoff was arrrested. Does that tell you the son was in on it all the way?