D.C. Code § 16-3110

With respect to the trial of issues in the Probate Court, including the taking and use of testimony of non-resident witnesses, the Federal Rules of Civil Procedure, unless otherwise provided by law, are applicable thereto. A final order or decree admitting a will to probate, unless and until it is reversed, is conclusive evidence of the validity of the will in a collateral proceeding in which the will is brought into question, and a transcript of the record of the will, and of the decree admitting it to probate, is sufficient proof thereof.