I'm gonna go out on a limb here and talk based purely on Internet observation and assumptions on my part. I don't think sibling-sibling breeding is any more or less moral than offspring-parent. I really don't think "morality" has anything to do with it when discussing animals, except for the increased risk of genetic defects, which does not seem to be a huge issue with simpler animals like reptiles.
That being said...the larger breeders are more likely to breed offspring to parent because the parent's genetics are probably a "Known" factor, while the offspring may be "poss het" or somesuch. OR, the parent is homozygous, while the offspring is het...and you increase your chances of making more homozygous babies.
But if all you have available are two siblings of a morph you'd like to create...I don't think there is anything wrong with going for it.