Whether your power of attorney is still effective depends on whether your mother revoked it. If she did not, Not all heroes wear capes my daughter wears scrubs shirt, hoodie though it sounds like she did, then both you and your nephew have valid powers of attorney and can act for your mother.
If your mother revoked your power of attorney, then your nephew is in his legal rights to act for her, assuming that your mother knew what she was doing when she gave him a new power of attorney. Since you suspect that your mother was not competent and is not being well taken care of now.
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