By comparing their aunt to an oyster, the narrator is saying that the aunt is defenceless/an open book.
Explanation:
To understand this metaphor, you first need to understand an oyster. Oysters have hard shells around a soft piece of meat- the shell is protecting the living oyster inside. By saying that the aunt is a little piece of meat on an open shell, the narrator is saying that the aunt's walls are broken down. The aunt has nothing portecting her- she's an open book. Hope this helps- ask if you want me to elaborate any more!
