Indian Succession Act and Indian Evidence Act

I would be highly obliged and thankful from the bottom of my heart…. If my Ld. Lawyer’s fraternity would answer my few queries regarding the competency of attesting witness of the Will. *I representing the plaintiffs in this case*. Deceased mother (housewife) of two-plaintiffs and three-defendants executed *Two Separate Registered Wills before Sub-Registrar*. One Will is in favour of Defendant No. 1 and attesting witnesses are her Son and Daughter (Daughter is mentally unstable). Second Will is in favour of Defendant No. 3 and attesting witnesses are his son and his wife. The concept of proving the execution of the Will by way of calling one of the attesting witnesses as per Section 68 of Indian Evidence Act is clear to me, but my queries are as follows: (1) In First Will :- If one of the attesting witness is mentally unstable then court would rely on the another attesting witness (son of the beneficiary of the Will) *because as per the Section 63 of Indian Succession Act says that execution is valid only when Will is attested by two attesting witness*. In this case, Section 68 of Indian Evidence Act would come in way to safeguard the Defendant or not ? (2) In the Second Will :- If one of the attesting witness is wife of the beneficiary (Defendant No. 3) and another attesting witness is son of the beneficiary then what is about the concept of witnesses to be "disinterested"…. in other words, that they not stand to inherit under the terms of the Will. A beneficiary’s spouse may also be disqualified from serving as a witness. *Would Section 67 of Indian Succession Act would override the Section 120 of Indian Evidence Act ?*