S. 43 of WESA governs the issue of gifts to witnesses of a will and declares that such gifts are void, unless the court is satisfied that the will maker intended to make the gift to the person even though the beneficiary or his or her spouse was a witness to the will.
43 (1) Unless a court otherwise declares under subsection (4), a gift in a will is void if it is to
(a) a witness to the will-maker’s signature or to the spouse of that witness,
(b) a person signing the will by the will-maker’s direction, or the spouse of the person signing, or
(c) a person claiming under a person, other than the will-maker, referred to in paragraph (a) or (b).
(2) For the purposes of subsection (1), the relevant time for determining whether one person is the spouse of another is the time when the will was made.
(3) If a gift is void under subsection (1), the remainder of the will is not affected.
(4) On application, the court may declare that a gift to a person referred to in subsection (1) is not void and is to take effect, if the court is satisfied that the will-maker intended to make the gift to the person even though the person or his or her spouse was a witness to the will.
(5) Extrinsic evidence is admissible for the purposes of establishing the will-maker’s intention under subsection (4).
An express gift to an adult child would not be invalidated if their parent is a witness. It is only a witness or spouse of a witness whose gift gets invalidated.
It is likely that a statement of such intention in the will itself would provide adequate evidence of that intention to satisfy a court to declare the gift valid.