The first post is obviously the first step. However I can't help wondering if the person giving the POA (donor) might now be unavailable or not competent to answer, and if your brother is refusing to co-operate. In that case you could think about how the POA might already have been used, for example to place the donor in an institution, to operate a bank account, or to contact the Government about something. This would probably have taken place after the donor lost competence. You could try to ask the institution, and to use any 'freedom of information' options that might give you a right to the information. Applying for guardianship yourself might force the issue. If none of these are appropriate you need to see a lawyer - it's a tricky situation.
Is your brother trying to stop her from doing something she wants to do, claiming that he has her POA?
If Mom wants to, she can assign a new POA, whether she has already assigned it to him or not. The document should say she is revoking any previous POAs and assigning the role to you (for example). It needs to be notarized. States have slightly different rules, and you can find them on the internet.
POA has limited authority. Even if he has it, your brother could not determine where your mother lives, and he could not stop her from spending money as she wishes. It would take guardianship to do those things.
What is the problem you are trying to address in finding out about the POA?
If Mom wants to, she can assign a new POA, whether she has already assigned it to him or not. The document should say she is revoking any previous POAs and assigning the role to you (for example). It needs to be notarized. States have slightly different rules, and you can find them on the internet.
POA has limited authority. Even if he has it, your brother could not determine where your mother lives, and he could not stop her from spending money as she wishes. It would take guardianship to do those things.
What is the problem you are trying to address in finding out about the POA?