Can POA be signed over to another person with out the POA's knowledge or releasing the responsiblitity?

A competent person can change her POA at will. It is sensible to notify the previous POA, but I don't think it is a legal requirement. This may vary by state.

An incompetent (in the legal sense) person may not be legally able to change the POA.

Your sister should have no problem showing you the document naming her POA. Start with that.

Has your mother been diagnosed as mentally incapacitated? I have read about this happening with others. If the POA you have was drawn up by your mother's elder law attorney or any attorney, I would go them for help. From what I understand, when changing POA from one person to another, there is no legal obligation to inform you that your duties have been revoked. If your mother has been medically documented as incapacitated, this may work in your favor if you chose to get a guardianship. I am sure others will post with more info for you.

Tell your sibling you would like to see the legal document that shows she is POA. It is possible to have 2 POA's, although personally, I would terminate my power if this were the case.