Of course there's still a lot to be revealed, but right now it seems like Sarah and Helena are the oldest to me. I'm pretty sure it was mentioned that Sarah's the only one who's not infertile (though probably not Helena, who I presume is not a clone). Helena is Sarah's
mirror twin, meaning their organs, handedness, hemispheric dominance, etc., are reversed. The difference between Helena and all the others is presumably why they think she's the original and the others are "evil," or the dark to her light, or whatever. If that's true then it makes sense that all the other "abominations" are cloned from Sarah, leaving Helena "uncorrupted." Like the incidence of infertility in the clones, I think the lung disease may also be an error in the cloning process (chronic health problems are common in cloned animals). If so, the progression of the illness is a good metric for judging the age of the clones relative to each other, but the "original" (Sarah) and her natural mirror twin wouldn't be expected to suffer from it.
Granted, I've watched a lot of these episodes inebriated.
EDIT: Well, I just watched the latest episode and Helena was described as a "creation of man." This implies they only told her she was the "original." Maybe she was artificially induced to split from Sarah in utero to become her mirror image twin or something -- just trying to hold the above theory together, really.