You may need to go back to the breeder of these cats and look at pedigrees in depth. Sometimes one ancestor passes along a specific gene that suddenly shows up down the generations. You could find that each of these cats, though seemingly not closely related, had the same ancestor(s) back a ways on both sides of their pedigrees. For an example I've seen descendents of a single litter bred out to other lines but down the generations someone uses those offspring for a litter not knowing that the cats way back all came from the same litter or a repeat breeding.
You might also want to make sure that the food that breeder fed the queens and kittens was not one with any melamine issues.
If you feed primarily dry it could be these cats just were not as good at drinking fluids as your siamese were. Cats being desert animals often don't drink much and when they eat dry food they may not have enough fluids to keep the body going right.
I'm sure you have been totally wracking your brain with all these dreadful losses but were there treats or toys these cats liked the siamese did not, did they sleep in areas not enjoyed by the siamese, did they chew house plants at all and so on. See if you can thing of anything that was different between the siamese and the orientals for daily habits etc.
If the orientals were more prone to dental disease the bacteria from that can impact the kidneys. I've been suspicous of shared water bowls possibly passing oral bacteria around too. Some cats are hit harder by that than others.
I hope this helps you and my deepest sympathies on your losses.
Cat Health, Behavior, Care Expert
30+ years cat owner, rescue, breeding, study of behavior & health care
Stress is always a factor in health and that could contribute.
If any of the orientals had bad teeth and they didn't share water with the siamese then dental bacteria is a possibility.
It may be the orientals got some exposure to a toxin.
If you live where radon is a possibility in cellar areas (and downstairs is a cellar or partly below ground) this info may be useful
http://www.epa.gov/ttn/atw/hlthef/radionuc.html
Dental infection is mentioned here as a top contributor to renal failure
http://www.pets911.com/health/vet-corner/chronic-renal-failure-in-cats/
more here
http://www.felinecrf.org/related_diseases.htm#dental_problems
My personal observation is that there appears to be potential for the dental bacteria to be spread pet to pet. I've seen several companies come out with a water additive to help reduce dental bacteria issues.
http://www.oxyfresh.com/pet/petoralhygiene.asp
You might just need to disinfect them. Or use an oxyfresh type product or something - they do work well to get cats to drink - such a dilemma!
I'll close the question after this reply to hopefully limit site emails. You can reply on it any time even so!