The primary risk is that minor genetic mutations may become expressed--in other words, a hidden deformity could be brought to light.
Reptiles in general seem very resistant to the ill effects of inbreeding, however--much more so than mammals. If neither animal does carry any problem genes, then breeding siblings won't cause any problems. It should not be done repeatedly over generations, because this increases the risks--as the parents become more and more similar genetically, small errors will become magnified until you inevitably WILL get noticeable genetic problems.
But for just one generation? Little risk is involved.
Line breeding, which involves breeding a son or daughter back to its parent, is a standard practice for 'fixing' a trait in animal breeding. We use this to prove traits to be genetic, and force them to express. (Breeding a het daughter back to a homozygous father to produce more homozygous animals, for example). Like inbreeding, it should not be done repeatedly over generations.
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