Other family members who reasonably or presumably would have inherited had anything been left (and so have an economic stake in what happens; i.e. have "standing" to sue, to use the legal term) can sue the POA sister, as can the executor administratory of the estate, who acts on behalf of the state and all heirs/beneficiaries. An attorney-in-fact or agent (those are the correct terms for the person granted authority by a POA) has a fiduciary duty to the principal (the person giving her the power) to act in accordance with the principal's best interests and wishes, and to NOT engage in "self-dealing," or taking advantage for herself. What you describe certainly appears to be self-dealing--acting in her own interest, not in the mother's interest or in accordance with her wishes--and so may be a violation of fiduciary duty. If a court agrees this was a breach of fiduciary duty, the agent can be forced to return everything she took. If you or others want to consider this option, you should consult with a trusts and estates attorney.