I am only aware one example invovling mere mortals, Oedipus Rex, and that was accidental.
The typical example was a god marrying his sister, but that was because if you were a god, and you needed to marry a goddess, your choices were really limited. Also, the gods of Greek myths were often the personification of forces nature - the brother and sister symbolically represented the male and femal elements, and it made it more symetrical for sibling to marry sibling. But it is uncommon outside of thr gods in Greek mythology.
You see this in other cultures, gods marrying their sisters. No other person would be good enough for thr god to marry. In the Norse Volsunga Saga, Signy is married to a man who had killed all her family except one brother, Signmund, who she helped escape. She sends all her sons to her brother to be tested, but they all fail, and she has Sigmund kill them because they are too week, Only the son she sired with her brothet (she had exhanged shape with a sorceress to sleep with him) proved strong enough for her task of revenge, i.e., only the pure blood of the Volsunga was strong enough. Same principle applied I think when Greek gods married their own siblings.
Bit when it