It is generally very dificult to prove undue influence by direct evidence. Rather, it is shown by facts and circumstances surrounding the decedent at, or around, the time that he or she executed the will. The more that you can "pin badges of undue influence" on the alleged infuluencer, the better your case will be.
Among these "badges," are creating a relationship of dependence or fear that the decedent would be abandoned; isolating the decedent from his family members; having the will prepared by the alleged influencer's lawyer; a significant change in the will in the alleged influencer's favor which differs from an established pattern in prior wills.
In some situations, the law will presume that there has been undue influence. These are where the alleged influencer is in a position of "trust and confidence" to the decedent. This mostly arises in the context of physicans, nurses, lawyers, clergy and other professionals. It can sometimes arise in the context of a 2d marriage.
You really need to consult a competent estate and trust lawyer in your state. You can find one by going to the web site of the American College of Trust and Estate Counsel, www.actec.org. Do not rely on advertisements to find a lawyer.
Fifteen years ago I remember a case that the person held a loaded pistol to the head of the testator and forced him to sign the will changing the beneficiary. When this came out, the contest for undue influence was easily decided.
In Florida a presumption of undue influence is created if the person who stands to gain under the will had a confidential relationship with the testator; actively procured the Will; and is a substantial beneficiary under the Will. I recommend you consult a local probate lawyer to discuss what evidence you might have against the daughter.
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