A power of attorney does not let someone control the principal (person granting the power) or prevent them from creating or signing a will (or contract or another POA--i.e. from creating or signing any legal document). As long as your mother is mentally compentent and does it of her own free will, she can create a will--even if it's a bad idea for her. If not mentally competent, or if tricked or coerced into making the will, it is possible that the will could be challenged on those grounds.