Check your document to see if it requires a doctor to certify that your sister is in fact incapacitated. If so, you are dealing with a springing document, which means you’ll need to give it to her doctor and request written certification of incapacity. Take the document itself (plus the certification, if you’re dealing with a springing power) to the landlord. The document should look rather formal (it will have a notary’s seal).
Also bring a copy of the document to leave with the landlord. (The landlord may want to show it to a lawyer.) You might also bring some explanatory material that will help the landlord understand the purpose of the document and feel comfortable honoring it. (You can get state-specific explanations of powers of attorney from this site.) Hopefully, this will convince the landlord that you have the legal right to enter.
Janet Portman is an attorney and managing editor at Nolo. She specializes in landlord/tenant law and is co-author of “Every Landlord’s Legal Guide” and “Every Tenant’s Legal Guide.” She can be reached at email@example.com. Copyright 2009 Janet Portman