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The insurance and 401k go to the designated beneficiaries and nothing will change that. To get your father's property, the woman would have to prove a common law marriage by proving the parties intend to be married, lived together as man and wife, and held themselves out to the public as man and wife. No set length of time is required. Your burden is to prove that at least one of those things did not happen. You could do that with live testimony of a disinterested person, writings from your father, video tapes, etc.
All of that will have to be fought out in the probate court unless you can find a number that will satisfy the woman enough to go away. You need a probate attorney to handle the court proceedings and to draft a proper settlement and release when or if the time comes.
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