There are two separate questions here. One is: for a given pair of closely related people who very much want to have sex with one another, is doing so (carefully) worth the risk? The other is: should we adjust our societal norms to make things easier for people in that situation?

It seems quite plausible to me that the answers might be "yes, sure" and "heck no", respectively because, as you say, if lots of siblings or other closely related people have sex then some of them will have children.

Slightly-parallel question: if someone is addicted to heroin and can procure some, should they take it? The answer might be yes, at least some of the time, but we probably still want norms that discourage people from getting addicted in the first place.

(I wonder whether laws and other norms against incest provide some protection against abuse by parents and elder siblings. That shouldn't be necessary -- they should be protected by laws against abuse and against sex with people almost certainly too young for properly informed consent -- but maybe there's some extra deterrent effect.)