I usually hate tsunderes, but somehow I really like this story. Mostly because the sister is such a fucking loser with horrible taste. The hypnotism/grievous bodily harm angle is normally something that'd cause me to drop this, but the fact that she uses it to live out these cringeworthy fantasies and embarrass herself somehow makes up for it.
I can only hope the childhood friend sees through this charade and uses it to humiliate the sister even further, for karmic justice.