Breeding of same mother cats

sometime's things happen.... especially with kittens. you could have the best parents from different family's and still something could go wrong.

this paragraph is from this web page :http://cats.suite101.com/article.cfm/inbreeding_cats

"inbreeding happens naturally in the wild between cats (or any animal) that live in one general area and have a small selection of breeding males, who continue to mate with siblings, as well as their own offspring, depending on the population and circumstances. This does cause problems in the wild. Many species are facing extinction due to low birth rates, weak immune systems, inherited illness, and other problems."

I'm not sure what happened to your kittens, and can't say for certain whether inbreeding played a part in your particular scenario, but I do know that, while it's not the worst thing ever, it's generally not a good idea to breed such closely-related animals. Do you know if the father cat was different for each of the previous two litters? At least that way, the current breeding pair would have some disparate genes.

In breeding in animal nature is normal. Don't worry. It happened with my cats and the kittens turned out just fine! Good luck in the future!