I am slightly confused by this question. I don't actually think that Isabella is given much power in the play, and I don't think she develops in this regard during the play either. She appears to be manipulated and used at every turn, even by her brother, who tries to get his sister to sleep with Angelo to save his life. The only time she is given the opportunity to have power over another is in Act V, when she publicly denounces Angelo for his actions and calls him a "virgin-violator," yet it is important to remember that this is an opportunity that is given to her by the Duke and is not something that she does of her own volition. At every stage, Isabella is shown to be living in a world where her powerlessness and vulnerability as a woman is exposed. The much-debated ending, where the Duke takes her as his wife, can be used to support this. For Isabella, who has done everything she could to defend her chastity and her desire to be a nun throughout the entire play, to suddenly and quietly acquiesce to marriage to a man that has manipulated her just as he has manipulated others indicates her lack of power and social standing. This play presents us with a man's world in which women must do what they can to survive, whatever the cost.