I suppose if someone wishes to do these things then they should make their reasons for doing so clear, but preferably/always after taking legal advice.
In this particular case, the person who is alleged to have signed the so-called Form, relinquishing her rights as a beneficiary, claims they did not sign it but remembers being called to her sister's solicitor's office years ago to sign some family paperwork.
It was only some time after her sister's death (the testator had died some years previously leaving everything to that sister, apparently on the basis of the signed Form) that she looked into the matter and realised that she was not a beneficiary (whereas she certainly was a beneficiary on the Will), and after a fair bit of digging, the Form was shown to her. "You signed" is about all she was told by her sister's solicitor.
It is not a simple matter, and I suppose if these things were simple there would be no need for lawyers (or indeed internet forums such as this!)
So I wonder, would it be unusual for the ultimate beneficary's lawyer to arrange and be the sole witness to such an event, and would that provide sufficient authority for the executor to act as if the Will had been amended as to exclude one of the sisters because she had apparently signed a form to be excluded?