Location: East Wing, East Tower (Guarded by Desmodaui)
Rescue Reward: Buster's Cage key
From the Manual: Your mother’s sister used to be a lion of the London high society, but has lived an easy life after the death of her husband a couple of years ago. She has been invited by Rebecca to keep a watchful eye on Buster until your arrival.
Unlike most of your relatives, Aunt Sophie isn't randomly placed in one of the Castle's rooms. Instead, she's dangling from a chain at the top of the East Wing Tower. You'll find her when you enter the Tower and take the elevator up to the top. To free her, you'll have to climb up to the Tower's roof and win the boss fight with Desmodaui.
After you defeat the boss vampire and stake him in his coffin, just use the key you got from him on the roof's winch mechanism to lower Sophie to the safety of the elevator. Once you bring Sophie back to the Sanctuary, she'll reward you with the key to Buster's cage, allowing you to finally go and free your dog Buster.