(a) A witness to an attested written or a nuncupative will, to whom or to whose spouse a beneficial interest in property, or a power of appointment with respect thereto, is given by the will, is nevertheless a competent witness to the will and is competent to prove the execution or validity thereof. However, if there are not at least two other witnesses to the will who are disinterested, the interested witness and the interested witness's spouse and anyone claiming under the interested witness shall take nothing under the will, and so far only as their interests are concerned the will is void.
(b) A beneficiary under a holographic will may testify to such competent, relevant and material facts as tend to establish such holographic will as a valid will without rendering void the benefits to be received by the beneficiary thereunder.