In all pre-contact Polynesian societies, brothers and sisters were considered to have a special relationship. The brother was responsible for his sister's support and protection. If she quarreled with her husband, she could return to her home and her brother would have to take care of her. Her children had special claims upon their uncle.

In Tonga, the brother-sister bond is one of respect and mutual protection. One sign of this respect is that if they find themselves together, they cannot speak of any sexual matters. This brother-sister relationship is not limited to full siblings, by Western reckoning, but extends to cousins and second-cousins.

If a boy happens to find himself in a group of young people which includes a girl who is his classificatory sister, he may be the butt of much joking. The others can freely talk about his sexual exploits but he cannot speak up to defend himself.

In some special cases, this taboo could be over-ridden by political considerations. In some island groups, the high chiefs were considered so exalted that no one save a relative was of sufficiently high rank to bear chiefly children. Hence some chiefs had sexual relations with their sisters, or other close relatives; their children would have supremely high rank.