Sexual relations between close relatives.
Incest is strongly forbidden in Leviticus 18. Leviticus 20 even says some kinds of incest should be punished by death. The Bible treats incest as a very serious offense, calling it dishonorable and perverted.
Examples of incest in the Bible show it comes from bad character:
Lot's daughters got their father drunk and slept with him. Both became pregnant (Genesis 19:30–38).
Amnon tricked and forced himself on his half-sister Tamar (2 Samuel 13:1–22) .
Paul strongly criticized incest happening in the Corinthian church (1 Corinthians 5:1–5), showing it was still wrong in New Testament times.
Blood relationship, or consanguinity, is one reason for illegal sexual contact. This applies, for example, to brothers and sisters, parents and children, grandparents and grandchildren, as well as some aunts, uncles, nieces, and nephews. Leviticus 18 also forbids sex with relatives by marriage (affinity) and some aunts and uncles. In ancient Israel, there was one exception: a man could marry his dead brother's widow if she had no son (Deuteronomy 25:5–10).
While there are genetic reasons to avoid incest among blood relatives, the main problem is that it hurts families. Since families are central to God's work on earth, he judges incest very harshly. Families can't survive when there's sexual misconduct among their members.