"If All Men Were Brothers, Would You Let One Marry Your Sister?" is a science fiction short story by Theodore Sturgeon. It first appeared in Harlan Ellison's anthology Dangerous Visions in 1967.
Plot
It is about an Earthman who comes to the planet Vexvelt, shunned by the rest of colonized universe for unknown reasons. He finds it a utopian paradise, but then discovers to his shock and horror that incest is actively encouraged there. When it is shown that the reason that the planet is a paradise is because of this encouragement of incest (or, rather, the non-suppression of sexual instincts and urges including incest), then questions are raised of the reader as to how we face "unpleasant" issues.
References
If All Men Were Brothers, Would You Let One Marry Your Sister? Wikipedia(Text) CC BY-SA