What is the test for determining whether a deceased's will is valid, rational, or genuine?

… The legally significant finding in terms of the trial judge’s reasoning, however, was that the explanation given in her will was “valid and rational” (see para. 76), or “genuine and valid” (see para. 83). I agree with the trial judge’s comment at para. 84 that this was a “relevant circumstance”. Given this, and given the fact that the plaintiffs did not pursue their cross appeal, it is not necessary for us to consider whether it is in law determinative of what a fair and judicious parent would have thought appropriate. (See McBride v. Voth, supra, at paras. 135-42.) That issue, if it is seen as one, must await another day. [Emphasis in original.]