Inbreeding, especially brother and sister, will increase the possibilities of genetic faults. It will also emphasize the strong points of the line. I would not take that chance. I'm glad there appear to be no problems. I can only hope that no problems will appear in the future.
Ususally the repercussions of a inbreeding doesn't show after just one generation, most kittens by such a mating are normal, if you are not unlucky enough to mate to siblings that both carry a recessive gene for some desease, and the kitten inherites it by both parents.
It is when you start to make these close related mating over and over in a breed that you can se negative results, the breeds gene pool is degenerated, and you start to having health problems in the breed, such as bad fertility, small litters, bad immune system etc.