If your sister took advantage of your father's age or medical condition to take money for her own use without his knowledge or consent, she may be civilly and criminally liable for elder financial abuse. California has laws that would permit your father to recover the missing funds from your sister. If your father is competent to revoke the POA, this should be done at once to prevent further pilfering by your sister. I highly recommend that you contact an elder law attorney for assistance. Good luck to you.
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She cannot right herself checks without justifying for what use the money was used. If it wasn't used for your father's benefit than the act is criminal. You need to consult a lawyer to revoke her POA as soon as possible.