- Incest between parent and child (underage) is a crime and an ugly one at that. Unfortunately, incest between father and daughter (usually violent) used to be the most common type.
- Incest between parent and adult child is just plain weird.
- Incest between siblings is (outside of hentai animes) quite ugly, usually an older brother will force and abuse a younger sister (usually both underage). However, using imagination here, there might be "happy couples" out there and as long as they don't have children, it's.. well, who am I to judge?
- Cousins (first degree) are ok, but again, only as long as there are no children. Unfortunately quite many countries permit marriage. However if the cousin is from the mother/grandmother side (even second degree), it's extremely dangerous.