Hello everyone, thank you for your help in finding this story, although I'm not sure if it's still on this site or somewhere else.
The story is about an aunt who arrives at her twin sister's house and her son's. Once there, she seduces the nephew and pretends to be his mother during sex. During this time, we discover that the aunt was in a psychiatric hospital in the past for unknown reasons, or at least I don't remember them. The aunt's ultimate goal is to enslave her sister and take her place, and she manipulates her nephew into agreeing, turning her into her sex slave. The story ends leaving us wondering if they carried out this crazy plan.
I hope someone knows this story and can point me in the right direction. I've been searching for two months.
(I'm using Google Translate because I'm not a native English speaker.)