The "incest" taboo has been very flexible over time

[...] [T]he Trobriand incest taboo had a sharp matrilineal bias. Where sex with a mother's sister's daughter was regarded as "a real crime" and could "lead to consequences as serious as suicide," sex with a father's sister's daughter was approved to the extent of being recommended for inexperienced boys. In the Trobriand view she was "the prototype of the lawful, sexually recommended woman."

In the Trobriand view, "the sister [is] for her brother the very centre of all that is sexually forbidden [...]." When brother and sister had to appear in the same company [...] a rigidity of behavior and a sobriety in conversation were required of all those present. "No cheerful company, no festive entertainment, therefore, is allowed to include brother and sister, since their simultaneous presence would throw a blight on pleasure and would chill gaiety."

Where brother and sister were subject to the Supreme Taboo, father and daughter were free to interact frequently and casually. "Although father-to-daughter incest is regarded as bad, it is not described by the [Trobriand word for "incest"] [...]. The reason, for Trobrianders, is that while a woman's brother belongs to her clan, her father does not. He is only her mother's husband.

- Arthur P. Wolf, Incest Avoidance and the Incest Taboos