Actually the niece does know how horrible he is, she stated that a couple or so chapters back he can be scary but what he does is “necessary” and she’s okay with what he does especially now because she is obsessed with Shin. And she can use it to get Shin near her. She’s trying to force her way into Shin’s life after he kindly rejected her. She’s literally stalking him. Using people to dig thru his trash, finding reasons to occupy his space, trying to get him into her company, approaching his friends, putting on a two faced innocent act, getting him drunk, having him photographed with her purposely, looking into his personal life. These are her actions. Her uncle is bad but SHE’S CRAZY.

it must hurt a lot to be left in the dark about things this serious, especially when it comes to people you care about.
Also about that asshole's niece, even if she fucks things up I don't think I'll ever badmouth her even a bit considering she has no idea how horrible and manipulative her uncle actually is.
Anyway this is gonna get so bad before it gets better. Good luck