for a power of attorney to be valid, the person signing it must not be incapacitated and must be aware of what she is doing. Normally a power of attorney is signed in front of a notary and two witnesses. If you know who they are you might want to talk to the notary and the witnesses to get their opinion as to your mother's state a the time she signed the power. You should talk to an attorney about this.
You have serious questions here. You should seek legal advice. Besides the legal capacity issue at the time your mom signed the POA you may also want to explore if she is being exploited in any way. If you cannot resolve things amicably, you may need to begin a guardianship proceeding (where you could perhaps manage her affairs, revoking any prior power of attorney).
This is not legal advice nor intended to create an attorney-client relationship. The information provided here is informational in nature only. This attorney may not be licensed in the jurisdiction which you have a question about so the answer could be only general in nature. Visit Steve Zelinger's website: http://www.stevenzelinger.com/
I agree with my colleagues that you will need an attorney to assist you. You may want to have guardianship to address any depletion of your mother's assets under the POA held by your sister. Any actions done under POA must be in the best interest of your mother. If not, legal action can be taken against your sister for failure to meet her fiduciary responsibility. Of course you have the issue of whether your mother even had capacity at the time of executing the POA.
The information you obtain herein is not, nor is it intended to be, legal advice. It does not establish an Attorney/Client relationship. You should consult an experienced probate or estate attorney for individual advice regarding your own situation.