Attorney-in-fact for mother transfers title of mother's house to herself, the attorney-in-fact.
In this response, I intend to give legal information, not legal advice. For legal advice, you must consult an attorney who is licensed to practice law in your state, is free of any conflict of interest, and is available to meet with you regarding your situation.
Your sister is the agent or attorney-in-fact for your mother under a power of attorney executed by your mother; your mother now has Alzheimer's disease and is incompetent; using her status as attorney-in-fact for your mother, your sister has transferred title to your mother's house to herself. These are the facts you allege. You don't say where everyone lives, but I will assume that all actors reside in North Carolina.
As a general rule, a conservator - a court-appointed fiduciary for a person who is incompetent or financially incapable - has legal authority that is superior to the legal authority of an attorney-in-fact and has to authority to investigate past and to ask the court to unwind prior transaction such as that in which your sister may have abused her authority when she transferred title to your mother's house to herself.
Attorneys-in-fact owe a duties of honesty and fair-dealing to their principal and may not deal with their principal's protperty in ways that enrich themselves.
You may be able to seek the appointment of yourself or a third-person as conservator of your mother's assets. You should consider reporting these incidents to an appropriate agency in North Carolina. Appropriate agencies include policy departments, many of whom now actively investigate allegations of elder abuse.