If the brother/sister-mating is the only inbreeding, it's not likely anything special wil happen at all, but there's always a risk involved when inbreeding- Inbreeding doubles the genes, the bad ones and the good ones. If the cats are healthy and there are no known hereditary diseases or defects behind them a brother/sister-mating PROBABLY won't do anything bad, bt I would never recommend breeding a cat with a relative closer than a cousin (cousins are to close if you ask me).
It's also important to remember not to inbreed inbred cats
My bredding queen will be mated to a stud who's parents are siblings (they have the same father, but different mothers). The males inbreeding coefficient is 12,5 % and that's very high. If you breed a male with his daughter the offspring will get an inbreeding coefficient of 25 %.
However my female and the inbred male aren't related, at all so their offspring will have an inbreeding coefficient of 0 %. This, outbreeding, is crucial to keep the breed healthy. You can't go on inbreeding on and on and on, because finally the breed will go into an inbreeding depression meaning physically smaller cats, numerous smaller litters and lower birth weights on the kittens and diseases and/or defects will start to pop up.
Therefor, no inbreeding is a good thumb rule