Breeding siblings will increase the chance of fatal genetic problems in the offspring. Inbreeding of close relations should be avoided. The more times you cross back into the same line the greater the chance of encountering unfavorable recessive genes.
It is unadvisable. There are significant genetic risks related to inbreeding (this has to do, especially, with recessive genes), which is why breeders selling M/F pairs of chameleons have to point out that they are "unrelated pairs". In the wild, it is not impossible for siblings to mate, but if the gene pool is diverse and the population big enough, the odds are more likely that they mate with unrelated animals
I wouldn't it's not a good idea with any animal. over time, inbreeding can reduce the genetic diversity and cause problems related to a too-small gene pool that can include an increased prevalence of genetic disorders.