How can I challenge and cancel the guardianship order legally?

My husband's sister deceitfully obtained a guardianship petition, claiming that he is psychologically unfit. He was admitted to a hospital and rehabilitation center for aftercare following treatment for a leg infection. Both he and I have been locked out of our residence, and she continues to provide false information about his discharge, stating he will be released in intervals of 15 to 30 days, yet he remains hospitalized. I have recently learned that he is receiving psychological treatment. What steps can I take to challenge and cancel the guardianship order in the High Court, and what other legal remedies are available to me?