You appear to be mixing apples and oranges. A will is witnessed; usually a trust is notarized.
This sounds like it was a "do-it-yourself" job; I can't imagine any competent lawyer would let a beneficiary's husband and daughter be witnesses to a will. It raises too many issues. California probate code section 6112 states that just because a witness is "interested" in the will does not automatically make the will invalid, but it does create a strong presumption that duress, undue influence, fraud, or "menace" was involved.
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Can you please clarify if you are asking about the witnesses who signed the WILL or the witnesses that signed the TRUST? You mention both in your question. Generally, the only people who sign a TRUST are the Grantors (people establishing the Trust) and the Trustees, along with the atttorney and the notary public. The WILL generally has to be witnessed by at least two, adult, related persons. It sounds like this person's Will was witnessed by her son-in-law and granddaughter along with a neighbor (if I am reading your facts correctly). If this is the case, it would be outside my comfort zone to have a son-in-law and granddaughter act as witnesses to the Will. The son-in-law might be far enough removed to act as a witness, but perhaps she should talk to her attorney about this issue and determine if she should re-execute the Will with only non-related parties acting as witnesses. Hope this helps.
The will is witnessed, and the trust is notarized. This all makes it likely that the person who signed was the person who was the testator. However as said above, if there person was weak, or sick or ill, the signor being a beneficiary could lead to presumption that they influced the signing.