The Sestra was originally the vessel of a civilian man who facied himself an 'Adventurer', and gathered a small civilian crew to go out adventuring with. During his adventuring, he eventually found the Gen'ei Gen'ei no Mi on the Island of Metal Teeth inside of a small box with a note reading 'Gen'ei Gen'ei no Mi: Do Not Eat. (This means you Aimi!)'. Curious, he ate it, and then made his way back to his ship and attempted to make a simulacrum, or a duplicate, of her (Yes, that was intentional- there is a very good reason for it though...)