A Stranger in the Family by Robert Barnard
Robert Barnard was an award-winning novelist well-known for his skill at plot-twisting "cosy" mysteries. A graduate of Balliol, he lived in Australia and Norway, published forty books, some under a nom de plume. He died in Leeds in September, aged 76.
This tale revolves around the identity of an adopted protagonist who finds out from his mother on her deathbed that he's not who he thought he was.
Kit was adopted when he was three and grew up as an only child in a happy home. His father, a Glasgow newspaper editor, was one of the Jewish children rescued from Nazi Germany by Kindertransport just before the war. His mother was a professor of Art History.
After his mother's deathbed revelation that he is adopted, Kit follows her instructions and discovers the name and address of his birth family in Leeds.
Kit winds up his mother's affairs, then takes a break from his studies at the University and leaves the family home he now owns in Glasgow to go to Leeds in search of his birth parents.
His mother's greeting is lukewarm, and Kit soon realizes she's keeping something from him. One of his newly discovered brothers is downright hostile to him, and his birth father, a Leeds lawyer now divorced, out of touch with his family, and suffering from Alzheimer's, has an altogether darker history Kit could have dreamed. That shadow implicates Kit's mother as well.
As it happens, the respectable and loving home provided by Kit's adopted parents comes from people with connections to the Sicilian Mafia.
In short, this novel sets a few old ideas on their ear. Blood does not tell, nor is it thicker than water. The apple falls a very far from the tree and nurture overcomes nature in the child-rearing stakes.