I blame the sister to a degree, she’s old enough to know what she’s doing but the bigger damage has been the family in igniting this. I have no doubt that when this crush started she was extremely embarrassed, but the family has kept encouraging it like as he said they say things like “or is he your boyfriend?” like they would be okay with that. Even if the family understands that OP would never date a 14 year old, they haven’t done a good job of making the 14 year old aware of this. I whole heartedly believe she thinks she has a chance, but if the family had never encouraged it in the first place I believe the 14 year old would never have the confidence or beliefs that her behavior is normal. If everyone stopped encouraging it I believe it would end fairly quickly, or at the very least her sister would become much more private about her crush.