Location: In the Cathedral
Rescue Reward: N/A
From the Manual: Your sister Rebecca is the oldest of your siblings and has had a hard time marrying someone of her social level in England, despite her looks, intelligence and social skills. Your sister Rebecca doesn't appear until the game's final battle. No matter what you do, her fate is sealed because she´s been sacrificed by The Count, so don't worry about her. Ingame quote: "Your sister is dead and there is nothing you can do to change that."
It can be assumed Rebecca's sacrifice is required. Without her soul being devoured and empowering Malachi, the fight would not occur and the vampire would have remained imprisoned.