| YOur husband would have to sue and prove that his father was incompetent AT THE TIME his father gave his aunt the medical power of attorney. Then he would be able to have that POA voided and THEN he could sue to be named his father's POA provided he has served all relevant parties (his aunt and most likely his father's other siblings).
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Parents should remember two things: Love your kids more than you hate your ex (or soon to be ex) and when you have children the relationship with the other parent is until death parts you.
Nothing stated by me should be taken as giving you legal advice or forming an attorney/client relationship. My advice is based on the law and not deemed to necessarily apply to the specifics of your case. The devil is in the details after all. | 
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