Oh, this is such a specific and dark trope to unpack. The yandere archetype is obsession personified, but the sister dynamic adds this intensely personal, domestic layer to the obsession. It's not just about a stranger wanting you; it's about someone who shares a history, a home, and a presumed right to your life twisting that into something possessive.
I think a core trait is a distorted concept of love and ownership, fused with a deep-seated insecurity. The 'bullying' isn't random cruelty; it's a method. By undermining your confidence, isolating you from others (friends, potential romantic partners), and establishing control through fear or humiliation, she's trying to reshape your world so that she is the only source of stability, validation, or even pain—all to make you utterly dependent on her.
There's also often a perceived betrayal at the root. Maybe you 'abandoned' her by growing up, seeking independence, or showing affection to someone else. The bullying is a punishment for that imagined betrayal and a frantic, messed-up attempt to force the relationship back into a bubble where she is your entire universe. The domestic setting makes the forced proximity inescapable, which is half the terrifying appeal of these stories.