Quote Originally Posted by Danny Beane
I am no expert on the matter so take this for what it's worth but I believe they mention it because the sibling could be Het for the same gene as his morph brother/sister since they are related.
A sibling to a Spider is not het for anything as Spider is a Dominant and not a recessive gene.

All it means is that the sire or the dam was a Spider and the other was a normal and the clutch produce contained Spider & Normal.

Now for the HET here is an example breeding an Albino X Het Albino would produce a Homozygous (Visual Albino) and Normal offspring looking considered het for Albino and it that case that is what they would be sold as 100% Het Albino and not Albino Sibling.

Hope this make sense.