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.
This is not legal advice nor intended to create an attorney-client relationship. The information provided here is informational in nature only. This attorney may not be licensed in the jurisdiction which you have a question about so the answer could be only general in nature. Visit Steve Zelinger's website: http://www.stevenzelinger.com/
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.
** LEGAL DISCLAIMER ** My response above is not legal advice and it does not establish an attoreny-client relationship. When responding to questions posted on Avvo, I provide a general purpose response based on California law as I am licensed in California. In reviewing my response, you are specifically advised that your use of, or reliance upon any response I provide is not advisable. I do not have all relevant background details or facts related to your issue / matter, thus I am not in a position to give you legal advice. Further, your review, use of, or reliance upon my response does not establish an attorney-client relationship between us nor does it qualify as a legal consultation for any purpose. For specific advice regarding your particular circumstances, you should consult and retain local counsel. Law Offices of Eric J. Gold www.EGoldLaw.com Telephone: 818-279-2737 Email: email@example.com