Marriage.

I doubt very much that is written into your POA document, which gives you rights to do what is enumerated in the document itself. Being unable to read and write doesn't mean one is incompetent to make choices such as marriage. Many able bodied make poor decisions as regards spouses.

As your sister may be debilitated enough to require guardianship it is time for you now to see an Attorney for legal advice. We cannot know her, you, or what is happening with her medical, mental condition.

I surely do wish you the best. This has to be terribly distressing. I am glad your Sis has someone on BOTH sides here concerned for well-being.

Your POA does not give u the right to stop her from getting married if she is not mentally challenged or diagnosed with Dementia. If she legally can't sign a contract, she can't marry.

Guardianship, she again has to be mentally challenged or have Dementia.

Yes, guardianship. Maybe contact social services to see if you can get emergency guardianship, or allow the county to become her guardian.

As POA no, you will need to file for Guardianship.