As you have no doubt observed, brothers and sisters do not mate.
While they may find one another attractive, they find the notion of sex with one another repulsive. This is because a child produced by siblings has a high probability of sharing the same recessive genes, and recessive genes are often harmful unless coupled with dominant genes.
Our sexual desires have been honed through evolutionary history so that we do not waste energy producing offspring that are unlikely to survive.
Lebanese and Chinese societies have attempted to arrange marriages between children raised together, but those marriages almost always do not work out. And, children raised together in Israeli kibbutzim do not marry, even though they are not discouraged from marrying.
On the other hand, when sibling incest does occur, it is almost always between brothers and sisters who were raised apart.
So, it's not that being a sibling is a turn-off, rather it's having been raised with another that does the job.
The situation is simular with sons and their mothers. While much of Freudian psychoanalytic theory is based the Oedipal complex, Steven Pinker and John Tooby point out that Oedipus was separated from his mother at birth, and Freud was raised by a wet nurse.
- How the Mind Works by Steven Pinker (2009 book) page 455-460, Pinker cites Tooby 1976, Brown 1991, Daly & Wilson 1988, Thornhill 1991, Ralls, Ballou & Templeton 1988, and Buss 1994
- "The evolutionary regulation of inbreeding" (1976 article) by John Tooby
- "The evolutionary psychology of incest avoidance" (1976 article) by John Tooby