Codswallop.
They were named the Seven Sisters by none other than that old romanaccio of Carletto Mazzone, who by doing so wanted to point out how (for a short moment) there was a huge gap (in terms of - mainly financial - power) between said Seven Sisters and the rest, which enabled them to dominate the other teams pretty much as they wished.
The expression itself was (unwillingly?) borrowed from the oil industry, and more precisely from the then President of ENI Enrico Mattei. According to him, the Seven Sisters were : Standard Oil Trust, Caspian and Black Sea Petroleum Company, Royal Dutch, Shell, Texaco, Gulf Oil Corporation and British Petroleum. Just like their football successors, these multinaltional companies were enjoying a situation of almost total monopoly in the late 60's/early 70's.