if they have bred they have a 50/50 chance they may be deformed or they may be ok .
i bred a half brother and sister (same mother different fathers) last year none of the kits were deformed and the kit i kept back is a cracking worker .
i will tell you the difference between line breeding and inbreeding . when the babies are normal its called line breeding when it goes tits up and they are born deformed its called inbreeding
its been going on for years ferrets , pedigree dogs , royalty lots of things are "inbred" .