Breeding siblings once is usually fine (even in humans the birth defect rate is still very low for children of siblings, only about double that of unrelated people), its a matter of how many generations the line has been inbred without outcrossing. Keep breeding siblings and eventually you get deformities (i.e. bend spines). Livebearer breeders keep two separate sets of the same kind and cross breed them every 2 or 3 generations.
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