No, saving Sarah wasn't a possibility, but saving Lee was. She had influence as Lord Voldemort's favorite. In the past, she'd used it to her advantage many times. Rarely had he ever forced her to do something she didn't wish to do. Denying her choice of husband had been one of the few times he hadn't allowed her to do what she wanted. If she could promise her husband that she would be the perfect, meek, helpless little wife he always longed for, maybe he would allow her to live long enough to beg a favor from their master. Getting Lee out of Azkaban and back to his girls was the absolute least she could do for the man whose life she ruined.