Incest is a word for any kind of sex between family members: parent and child, brother and sister, or cousin and cousin.
There are many disturbing things in the world, and incest is near the top of the list. Incest is when people who are related have sex. This could take many forms: a parent raping a child is incest. Two first cousins marrying is also incest. Different states have different rules for how distantly related two people need to be to avoid incest, mostly because children born from incest often have serious health problems.