Robert Durst, the New York real estate heir, has sued a brother and a cousin on allegations they were trying to restrict his right to leave assets to his wife in his will. Mr. Durst was found not guilty in November of intentionally killing his neighbor in Texas, but remains incarcerated in Galveston on bail-jumping and evidence-tampering charges. In papers filed yesterday in Federal District Court in Manhattan, Mr. Durst said his brother, Douglas, and cousin, Jonathan Durst, had put restrictions on trusts left to him by his father, after they learned in October 2001 that he had married Debrah Lee Charatan. A statement released by the Durst Organization said it had not seen the papers and would not comment. According to the lawsuit, the Durst family’s portfolio of New York office buildings is estimated to be worth billions of dollars.