ok, let's assume that one of the parents is het for a potentially harmful mutation. We don't know if it's the mother or the father. Now let's assume that the other parent is normal/normal for that same gene, otherwise it is probable that we would see at least one mutant in the orignal clutch. That means that a given offspring is 50% het for that mutation as you said. Any sibling to that particular animal would also be 50% het so a cross between two siblings would be a 50% het X 50% het meaning that there is a 25% chance that you would produce a clutch with 25% mutant animals.
Now, if we crossed that first offspring snake back to one of its parents it would be the 50% het offspring like before, and the parent would also be 50% het since we didn't know if it was the mother or the father that was carrying the mutation in the first place. So it still ends up being a 50% het to a 50% het by odds. Now if you knew for some reason which parent was the carrier (although I can't think of an instance offhand where you would), it would actually be better odds to cross the offspring back to the parent that wasn't the carrier than to a sibling.
Now, if someone has some emperical data that shows they've had better luck crossing siblings or parent/child, I can't really argue with that, but if you're looking purely at the numbers, they are the same.
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1.1 Red Bloods (Cartman and Bebe)
0.1 Boa Constrictor (Victoria)
0.1 Albino Burm (Butters)
0.0.1 Desert King (Miles)

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