Thanks for caring enough to take care for these poor strays. If the kittens are truely 10lbs, they are adults by now and the list is endless for what could cause an outdoor, unvaccinated cat to be sick. Deadly contagious disease like Feline Leukemia, FIV (cat form of HIV), and FIP (feline infectious peritonitis) are what come to mind first in this situation. If these cats were out side my house, I would start by blood testing the sick one first. If one has any of these diseases, than it is very possible that they all do. The problem with leaving them free to roam if they have one of these diseases, is the potential to infect other strays or even our own house pets, by rubbing on our screens in the summer or even leaving virus outside of our homes for us to bring in as we come in from the outdoors. This thin cat may also have some congenital or developmental problem that does not affect the other animals. A veterinary exam is the first step.
The other problem that I see here is the good possibility that these few cats will reproduce exponentially and within a short time (2 litters a year) be responsible more than a hundred stray cats. I would capture them, do whatever medical care I could afford (test, vaccinate, spay/neuter, deworm) and contact your local shelter to find them loving, indoor homes for the hollidays. Dr. Tony Kremer