Answer:
Incest is being sexually attracted to a close relative. This is not a fetish. (If you are sexually turned on by other people's incestuous relationship, that's a fetish!)
People usually attempt incestuous relationships because they are unable to develop sexual relationships with "strangers". They are usually predatory: Parent on child, older sibling on younger, etc. The aggressor is more interested in her or his own feelings than those of the other person. Counseling is strongly recommended!
When a family relationship turns sexual, it almost always ends up destroying the original relationship.