(a) A contract executed or entered into on or after September 1, 1979, to make a will or devise, or not to revoke a will or devise, may be established only by:
(1) a written agreement that is binding and enforceable; or
(2) a will stating:
(A) that a contract exists; and
(B) the material provisions of the contract.
(b) The execution of a joint will or reciprocal wills does not constitute by itself sufficient evidence of the existence of a contract.