Is she mentally ill? please help..?

someone very close tome in my family has for the last 10years shown quite weird behavior. every few months we have problems as she perceives things wrong. its always around me doing something horrible to her mentally, like imagining things like im after her boyfriend or i look angry and its to do with her. none of what she says is true and she gets all this just by making it up in her head. not by anything ive said. this makes things very difficult for me as i feel shes attacking me? what could this be as ive never done anything wrong but she always thinks i do and shes adiment i cannot change her mind? please help?