I suggest speaking with a local attorney who can review the papers you have.
If this person has no legal authority over your sister and you have been named your mother's Power of Attorney, on that theory, you should be able to go get your sister. However, the police likely won't enforce anything with out a signed Judge's order stating that you are the sole or managing conservator of the child.
You should have a local attorney review those papers to see what actions you can take. If you have full power of attorney to make all legal decisions, you could file in her stead and get a court order over your sister. The police don't like to get involved in this type of situation unless there is a court order.
Unless your mom emancipated your sister, her notarized signature probably doesn't mean a thing. If your mom did emancipate your sister, you have bigger problems - she can probably do as she wishes.
Disclaimer: This answer is intended for informational and educational purposes only and should not be considered legal advice nor forming the attorney client relationship.
I agree with Ms. Watson and Mr. Harding.
Contact a local attorney and bring all your papers with you. If this is a Tarrant County case, you should go to the Tarrant County Family Law Center (Family Law Courthouse) and ask to get an unofficial copy of the file concerning your sister, if there is one.
Also, go to the probate courts and see if there is a file on your mother or sister regarding guardianship.
Once you have all these papers (to the extent they exist), you are in a position to take them to attorney for review.
If you can't afford an attorney, call the Texas Legal Services Center (www.tlsc.org) and they will refer you to an attorney in your area who will agree to help you at a significant discount, if you meet TLSC's income guidelines.