She was raised as the daughter of Arthur Terrence O'Brien Thompson, a British mechanical engineer from Darlington who worked in Indian Railways and his wife, Charlotte Selby, a Eurasian from Ceylon (Sri Lanka). Her mother also had Māori ancestry. However, according to her birth certificate, Merle's biological mother was Charlotte's then-12-year-old daughter, Constance. Charlotte had herself given birth to Constance at the age of 14, the result of rape by Henry Alfred Selby, the Anglo-Irish foreman of a tea plantation. To avoid scandal, Charlotte raised Merle as Constance's half-sister.