POA is conferred by the principal (mom) and if she has the capacity to revoke it then she can assign POA to someone else. If she is too far gone with dementia to do that and is truly incapacitated, then the best way to resolve this may be through guardianship, if he is not acting in her best interests. However if you head into court and he is the AIF under the last valid POA then that will be a big factor for the judge to consider in possibly appointing HIM the guardian. So you need to hire a knowledgeable attorney who can help you think this out well in advance. James Frederick of Livonia is active on AVVO and is a fine lawyer with much expertise in this area. Good luck.
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As long as your mother has capacity, she can revoke her existing POA and sign a new one. Since it sounds like this is likely to be contested, I would strongly suggest that you involve an attorney to take care of this. There is a chance that if the family cannot agree on these issues, that the probate court may need to get involved. That is always unfortunate.
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In addition to the statements of my fellow attorneys regarding the revocable nature of the POA so long as your mother is mentally competent; I would also add that the answer may depend on the "type" of POA. Some key questions include:
1) Is the POA "Springing" or effective upon execution?
2) Is the POA for finances, medical, or both?
3) If there are seperate documents for finances and medical care, is the acting sibling also named as the POA for heatlhcare, a/k/a Patient Advocate?
4) Does the POA contain a "consultation" provision requiring your brother as POA to consult with / inform you and the other siblings regarding his actions as the POA,; and if so, is that provision required or precatory in nature?
Finally, if all else fails, you may encourage your sibling with the POA to consult with an Elder Law / Estate Planning Attorney. Depending on your mothers health and financial circumstances, there may be anumber of options, each with vastly different consequences available to you and you family.
Hope this helps.