I think they were all made of the Little Doctors to some extent. Roland had a sense of 'variousness' on looking at them. Something he didn't get from Jenna, who seemed almost just a human girl. (He turned out to be wrong, mind you.)
I wonder if this the same for all type 1 vampires or if the Little Sisters are a species to themselves. I would go for the latter although (later books spoilers)Spoiler: