The probate court in the County in which the decedent lived is the final decision maker on whether it is valid. There are fairly simple requirements in order to validate it and ultimately probate the estate. If there are potential issues with family members, you should contact an attorney to discuss this with in more detail. He or she will advise you if these family members even have the right to contest the will under NC Statutes and if so what their chances of prevailing might be.
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Older wills are generally valid as long as they meet the legal criteria to qualify as wills. If the will is valid then it will be followed and it will be unlikely that relatives not mentioned will inherit. However, if the will is not valid then Intestate Succession Laws will be followed and relatives may inherit depending on how closely they were related to the deceased. I would recommend that an attorney be hired in this situation.
Age alone cannot cause a Will to expire or be invalidated. To be a beneficiary under a Will a person does not have to be specifically named if they are within a class of people named -- such as "my siblings, my children, or my nieces". Also, it may be that your brother had assets that were not controlled by his Will. I encourage you to hire a lawyer to look into the validity of the Will as well.