Blindsight by Maurice Gee

Blindsight is the story of a good though damaged man and his less than virtuous sister. As their childhood closeness unravels, Alice moves into her career in science (she's a mycologist), while Gordon descents into vagrancy and silence. For more than thirty years they do not meet. Then a young man appears at Alice's door, claiming a relationship she never knew she had. As he becomes part of her carefully guarded world, she cautiously begins to reveal the past. But is she telling him everything
Jealousy, ambition and love shape the fates of Alice and Gordon in this compelling story of loyalty and family ties. Blindsight is another fine novel by the writer many acknowledge as New Zealand's best.