If you grew up in a household and society that approved of sibling sex, then you would probably do it and not think anything bad about it. But I don't know of any society (probably there are some somewhere) where that is considered appropriate, so most people would avoid that.
Many cultures accept family nudity. Siblings see each other nude, but they still do not have sexual contact. They may have a sexual interest in a sibling, mainly because it is their first encounter with the opposite gender in the nude, but they usually don't act on that interest.
In a society where family nudity is not the norm, there are still some households in which it is accepted. And even where it is not accepted by the family, the children may try to peek at siblings or even have mutually agreed sessions of revealing the body as a way to learn about the other sex (usually at a very young age). But a sexual encounter? Over the line.
A comment on sunseeker11's response: A woman of 20 talking a 13 year old into stripping and jacking off is way out of line. It is a criminal act! And doing it with one's own brother is worse. I understand (somewhat) a 13 year old getting into it, although I would have expected him to have had an interest in seeing her naked and trying to get her to reciprocate, but he did not mention anything like that. But although his experience does sometimes happen, it shouldn't!
Anyway, although it sometimes happens, it is not very often, so it is abnormal.