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Usually those don't terminate automatically after a predetermined time unless they were granted because the person's age.
As that is not relevant in your situation, I'm sure this guardianship will continue until you petition the court for it's termination.
So, you will have to file a petition for termination of the guardianship before the court (and judge if still sitting) who entered the original order.
Will your sister oppose the petition to terminate the guardianship?