“to uncover their nakedness.” This is an idiom for sexual intercourse. Up until the Mosaic Law, there was no regulation or law about marrying a close relative, and with good reason. After God created Adam and Eve, and they began to have children (Gen 5:4), the only persons available for someone to marry were a sibling or close relative. Furthermore, that same situation occurred again after Noah’s Flood. Also, people lived in family and clan groups, and so often most of the people in the group were related. Thus, for example, Abraham married his half-sister Sarah (Gen. 20:12). However, by the time this Law was given about marrying close relatives many centuries had passed since the Flood, and there was no need to marry a close relative.