NHDT-962 Problem: My best friend's little sister, who I've known since playhood, suddenly

A girl who only sees her as her best friend's little sister suddenly seduces her when she leaves her seat. When I was puzzled, this time I flipped my skirt and showed my bare legs, and while touching my crotch, I had a meaningful smile. My crotch begins to swell even though I am confused whether it is serious or a joke. You don't know when your best friend will come back What would you do in a situation like this?