Possessions and hauntings can occur when people mess aroung with occultic stuff - even if they don't really believe in it. The Exorcist is based on a true story - except the girl in the movie was actually a boy in real life. He got possessed after he did the Oujii board with his aunt - who was into the occult. Ghost sightings and parnormal ghost activities in a house are demonic in origin. Usually, if you do your history on the house, you will discover that someone who once lived there did something occultic, - such as having a seance to contact the dead, doing Oujii boards, having a psychic reading, etc... These activities, even when done for fun, can invited a demon or demons into your house. If you mother dabbles in that stuff, there is a possibility your house will become haunted (or she might get possessed or obsessed).
Fr. Amorth (the exorcist of Rome who wrote the book "An Exorcist Tells His Story") explains that proximity to evil places or persons can bring about possession. This includes attending spiritualistic session, dabbling in magic, or consulting magicians, witch doctors, and some card readers; also, practicing the occult, belonging to satanic sects, or practicing in rites that climax with black masses, and so on, put us at great risk.