I am sorry for your loss.
No power of attorney is valid after a person dies. Perhaps your sister-in-law is moving to be her husband's executrix or administratrix of his estate? That would make sense since she is entitled to have that kind of power as his surviving spouse. Should she be deciding to bring a wrongful death lawsuit, she would need to be appointed to act in that manner.
Many more details are necessary to advise you. Check with a lawyer in your locale to discuss more of the details.
Good luck to you.
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I agree with Atty. Brinkmeier. A "power of attorney" is void upon one's death. Your sister-in-law is probably petitioning to become "personal representative" of his estate. If your brother had a will, that will would name who he wanted as his personal representative. Otherwise, anyone who is a Florida resident, over 18 years of age, and who is not a convicted felon may apply to become the personal representative. Also, if one is related to the person who died (i.e, parent, brother, sister) an out-of-state resident can petition to become personal representative under Florida law. To answer your specific question, yes, you and your sister-in-law can petition the court to be "co-personal representatives"
A power of attorney is no longer valid after death. I would suspect his wife is giving your sister the right to act as Personal Representative on her behalf - however it would depend on if he left a will or not as to who gets the appointment. If you are in the Jacksonville area and need assistance, contact my office.