I just finished it and, wow, the conclusion really leans into its absurdist romance roots. The protagonist spends the whole story trying to 'bully' his obsessive, dangerous sister as she demands, which is a twisted sort of courtship in their world. The ending reveals their codependency is actually a sealed fate, a magical contract from a past life that binds them. Instead of breaking the cycle, they choose to fully embrace it. She officially becomes his guardian and, in a darkly sweet ceremony, he places a symbolic collar on her. It's not a 'they lived happily ever after' in a normal sense, but a 'they lived intensely together' in their own messed-up harmony.
The final scene is them walking into their shared future, with her promising to protect him from everyone, including himself, and him accepting that this obsessive love is his new normal. It’s a full-circle moment that validates the story's core joke—her yandere nature isn't a flaw to fix, but the central feature of their relationship. I found it strangely satisfying, precisely because it didn't try to sanitize their dynamic. The author committed to the bit.