Minako was the rightful leader of the Sailor Senshi. Though they often deferred to Sailor Moon, in her past life, there was no Sailor Moon. During the reign of the Moon Kingdom, Sailor Moon was Princess Serenity, whom Sailor Venus and the rest of the Sailor Senshi swore to protect with their lives.
Sailor Venus is the princess's decoy because they look so similar. If you saw Star Wars, the prequels that is, Padme Amidala had decoys as the queen of Naboo. The security measure. The manga actually says that Sailor Venus would frequently act as a Decoy for Princess Serenity.