I was thinking, could the sins be failed human transmutations? I know this sounds strange, but Lust used to be Scar's brother's girlfriend/sister/wife and he tried to revive her, but to no avail. Now that we have Sloth, who just so happens to look like Tricia Elric, and we know that the Elric brothers tried to revive her but failed. And inductively I think that Envy, Gluttony and all the rest of them used to be attempted human transmutations (Although why anyone would do such a thing for Gluttony is still beyond me, oh well, they must have their reasons). As for how they become Homunculuses...could it be that some of the 'price' is paid when the first attempt was made making a human, and then successively it was easier to make the same person? I dunno.
Another theory might be that Foenhime used to know all the people who are Sins right now, since it was hinted that he created them in the first place. He could have based them on the people he knew, or he knew was dead, anyways. There is a chance that Foenhime could have met Scar's brother (and got him into alchemy...?) and that Lust-prototype girl.
So, what do you think?