Adelaide Foyster and Adoption
Foyster and O'Neil, adoptive half-brother and -sister, are associated with Borley Rectory (see image), known as "the most haunted house in Britain." Adelaide is the adopted daughter of the Rev. Lionel and Mrs. Marianne Foyster; Vincent was adopted by Mrs. Foyster and her fourth husband, Robert Vincent O'Neil.
Adelaide, along with Mrs. Foyster, was the focus of the most famous episode of poltergeist activity in history, which lasted from October 1930 to October 1935, while the rectory was occupied by the Foysters. Although the house was haunted from the day it was built, these five years were the most active. In 1939 the house burned down and in 1944 the site was razed.
Vincent was not adopted until after the family had moved away from Borley, and did not know about its reputation until after his mother died in 1992. He has dedicated his life to documenting and analyzing the hauntings. He was born to a married English woman and an American soldier; her husband refused to accept him. Marianne and Robert O'Neil (coincidentally, another American) needed a baby to enable her to emigrate to the USA, and they came to an arrangement with his birth mother. Both Adelaide and Vincent have traced their birth families. (The full history of the family is extremely complicated, involving several apparently bigamous marriages and other adoptions, partially documented in reference two below.)
Another example of an adoptive child as the center of poltergeist activity is the case of the Luksanniemi Devil in Finland, around Easter 1886.
Banks, Ivan. The Enigma of Borley Rectory. (London: Foulsham & Co., Ltd., 1996) O'Neil, Vincent. "The Most Haunted Woman in England;" "The Ghosts that Will Not Die;" and "The Ghosts, Poltergeists and Phantoms of Borley Rectory, the Most Haunted House in England." All available at:  Tikkala, Heikki. "Finnish Poltergeists." Available at:  Underwood, Peter. The Ghosts of Borley: Annals of the Haunted Rectory. (Newton Abbot: David and Charles, 1973)