This fragment apparently refers to the relationship between Sappho's brother, Charaxus, and the hetaira Doricha. According to Herodotus (2.135), Sappho roundly abused her brother, on his return from Egypt (where Doricha had taken up residence), for his affair with her, evidently because he had been cheated by her out of a large sum of money. Athenaeus (13.596ab) says that Sappho reviled Doricha in her poetry. In any event, here the poetess asks Aphrodite to make sure the pair doesn't get back together.
