They could certainly try if they believe they were "disinherited" when the person changing the beneficiary did not have mental capacity. There may be ways to at least build a file that the person making the beneficiary change was okay when they made it. Generally however, it is hard to overturn a beneficiary designation without significant legal costs and proofs that are hard to bring.
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In Washington there is no requirement that you leave assets to all of your children, and if the parent had mental capacity at the time he made the beneficiary designation, then it was a valid designation. Unfortunately, whenever someone does this, it opens the capacity issue, particularly in the case where a few months later the parent was diagnosed with dementia. The child who was excluded has a right to contest the designation, and would argue that the parent did not have capacity.
Unfortunately, this kind of litigation can be ugly and expensive if the parent no longer has capacity. Keep in mind, however, that just because the parent has been diagnosed with dementia doesn't mean he doesn't have "legal" capacity which has particular standards. If you believe he does, it would probably be beneficial at this point to have a geropsychiatrist render an opinion. If you don't believe he has capacity at this point, then you have to do a cost benefit analysis on the risks and costs of standing your ground vs. finding a way to settle the matter.
My colleagues have offered sound advice. The question is whether or not the elderly parent was unduly influenced into disinheriting one of the children by excluding that child as a beneficiary of the retirement asset. A parent can affirmatively chose to not leave anything to any or all children. If, as you state, this parent was shortly after changing the beneficiary designation diagnosed with dementia, you may have a justifiable challenge. Bear in mind that such challenges are not easily proven and they can be very costly. You should consult with a local probate attorney for an in depth factual analysis.
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