Question

All cultures have taboos against ________, sexual relations with someone considered to be a close relative, although precisely what constitutes a close relative varies across cultures.

Answer 1

All cultures have taboos against incest, sexual relations with someone considered to be a close relative, although precisely what constitutes a close relative varies across cultures.

Answer 2

The way I understand the question is that it asks us to give a name for "
sexual relations with someone considered to be a close relative" - and the answer is "incest".

The probable reason why this incest arose is because people born out of such relationships often had genetic disorders, because the genes of their parents were too similar (it leaves less place for faulty genes to be corrected)