My husband and his sister are co-guardians. She moved out of state refusing to give up her rights. What can we do?

She does nothing to help. She criticizes every move we make. Recently she moved out of state refusing to give up her guardianship rights. She sold dad's car and later took money out of his account. She says it doesn't matter since all we have to do is say the car quit running and her lawyer heard dad say he wanted to help her financially. We don't agree with any of this and feel she should put all the money in the bank as it belongs to dad.