When you say you are "entitled," that would refer to anything left in your brother's "estate." Insurance proceeds pass by beneficiary designation and would not be part of your brother's estate. Once the proceeds are paid out, your claim would need to be against your sisters, on behalf of the estate. Anything you recovered would be partially divided by your sisters, as well. Unless you have compelling evidence to PROVE your brother lacked mental capacity when the beneficiary forms were changed, I am not sure this is a case I would pursue. You would have a very steep uphill battle. You would also need to retain a very good probate attorney to assist you. Under the circumstances, even figuring out what state to hire the attorney in would be an issue. My initial inclination would be PA.
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You need to have an attorney review the documents you have and obtain from you a detailed history of what happened when. It is likely that any litigation would occur in PA, but this is not certain. In PA there is no right to an inheritance other than for spouses and then only at set percentages. There is a further distinction between assets passing via will and assets passing outside the will or outside probate. For life insurance and similar assets such as retirement accounts like an IRA, whomever is listed as the beneficiary will receive the funds unless prior to distribution you can obtain an injunction or compel the insurance company to deposit the funds with the court for the judge to decide who should receive the funds. If funds have already been released to the alleged beneficiary, you may be able to still win a lawsuit and compel the person who received the funds to return them, but any litigation should occur as soon as possible. You need to have an attorney review your docments and history of events to determine precisely when the statute of limitations will expire.
If you brother died without a will (intestate), than you would be entitled to a share of his intestate estate if he has no children or spouse. That does not mean there is an estate. There is only an inheritance if there is probate property. Life insurance will go to the designated beneficiary and passes outside of an individual's estate. Life Insurance is not subject to inheritances taxes in PA. You raise the possibility of your brother being incapable of changing his designated beneficiary on his life insurance. That would be a very difficult case to prosecute. Sometimes you just have to let things go and move on, this could be one of those times.