Massachusetts General Laws Part II. Real and Personal Property and Domestic Relations (Ch. 183-210) Ch. 190B, § 2-505

(a) An individual generally competent to be a witness may act as a witness to a will.

(b) The signing of a will by an interested witness shall not invalidate the will or any provision of it except that a devise to a witness or a spouse of such witness shall be void unless there are 2 other subscribing witnesses to the will who are not similarly benefited thereunder or the interested witness establishes that the bequest was not inserted, and the will was not signed, as a result of fraud or undue influence by the witness.