Word Origin and History for seven sisters
"the Pleiades," early 15c. (see Pleiades), seven daughters of Atlas and Pleione, placed among the stars by Zeus. The Pleiades also are known as the Seven Stars (see seven). As a late-20c. name for the major multi-national petroleum companies, it is attested from 1962. They were listed in 1976 as Exxon, Mobil, Gulf, Standard Oil of California, Texaco, British Petroleum, and Royal Dutch Shell.