From what I have seen here, this can be a touchy subject for some people, and more often than not, incest is usually associated with abuse/rape so a lot of stigma ends up being attached to it.

However, it is not uncommon for mutual incest to occur derived from lack of parental attention, and a fairly normal curiosity of the opposite sex in younger children. Often a case of "I'll show you mine if you show me yours".

Honestly, I don't really have a problem with brother/sister incest myself under certain conditions. As long as it isn't rape or abuse, or a substitute for outside emotional attachments, a cover for a deeper psychosis and precautions are taken.