Brother x sister matings can be absolutely fine. Some lab strains have been bred brother to sister for decades with no outcross of any kind, and the mice are 99% clones now with no deformities or extra heads, just normal healthy mice. I've bred a few brother x sister rat and mouse litters and the litters are all fine, although I prefer to breed mum to son or daughter to dad. I do this for a long time and only outcross if I see a fault in physical form, fertility or temperament that needs correcting. No faults, no outcross.
However, I know the backgrounds of my breeding rodents, and I trust the breeders I obtained them from so I can be fairly sure that a inbreeding will not be harmful. If you don't know the background of your selected parents, then you could be fixing a hidden undesirable trait without knowing it as inbreeding can only work with the genes that are already there - if the genes are bad than bad offspring will result, if the genes are good, the babies will be good.
Sarah xxx