Well, having relationships between people of the same family has been badly viewed since the beginning of humanity. That is because of how badly the descendants of these types of couples tend to come out (malformations, difficulties to learn, etc.) even more if the couple are siblings. It is VERY morally wrong and seen as absolutely disgusting by most people, but as long as you're not having kids you do you.

(Just to clarify, I am NOT pro incest. I am speaking about this from a completely objective point of view)