HUGE PROBLEM, My brother sister cats mated!! What now???

if they jsut mated, you can get her spayed still for 3 weeks yet, even pregnant. Tell the vet she mated and she may be pregnant cause this could mean complications during the spay. If you plan on keeping them you dont need to worry much. Inbreeding isnt a big problem unless it continues to happen and even then you will only see weak, sickly kittens, with unusally large head, eyes and ears but not like what happens to us.