I was told in a world history survey course that in the case of Egyptian rulers marrying their siblings to maintain royal purity, one reason there was no kind of taboo (equivalent to the modern squick factor of 'marry my sibling? Ewwww!) in place for royal marriages is because brothers and sisters of the royal family were raised apart.
I'm curious if this is actually true, and if it is, what was the actual reason for raising siblings separately? Was it a strictly gendered thing--specifically raising the brothers and sisters apart--or were all the siblings raised separately? If it was gendered, was this done for the purpose of making marriage between siblings easier, or was there a different cultural reason for raising male and female children separately? (I assume that such a practice would not extend past the royal family, as I can't imagine the rank and file having the resources or the wherewithal to raise children apart).
Finally, if this is true, how did it actually work in practice? Would the brother and sister have any kind of familial relationship as we recognize it today, or would they regard each other more as...I don't know...business partners, who happened to share blood ties?
First of all, it is a myth, that, what we understand as pharaos and their queens, were siblings (except for perhaps one couple, but I think that one is up for discussion as well), though they were often related - cousins and such. Sibling-marriages only came in the Hellenistic Period after Alexander the Great's empire had fallen, and Egypt came under the rule of the Ptolemaic dynasty of Greek/Macedonian culture. This created a cultural/ethnical elite of Greeks/Macedonians and the local Egyptian people.
The Ptolemies frequently - almost always - married their siblings. This they did for (at least) two reasons: 1. They wanted to establish themselves as the rightful heirs to the pharaos, and the pharaos were divine, thus follows that the royal family is divine, and divine individuals could not simply marry mere mortals - and certainly not Egyptians, and 2. Another way to legitimize the Ptolemeis place on the throne was to mirror the sibling-marriage of Hera and Zeus, of Greek mythology, which made Ptolemy II and Arsinoë II (siblings) establish the cult of the 'Divine Siblings' (Theoi Adelphoi) around themselves. (I can recommend R. A. Hazzaard's 'Imagination of a Monarchy: Studies in Ptolemaic Propaganda' for more on this subject).
In Egyptian culture women enjoyed much more legal freedom, than Greek women did, which has been skillfully covered by P. W. Pestman in his work 'Marriage and Matrimonial Property in Ancient Egypt: A Contribution to Establishing the Legal Position of the Woman', and several other authors afterwards.
I am aware, that I did not answer to primary question. I think it is a hard question to answer to be frank, especially if you refere to - as I think you do - the Ancient Egyptians before the Ptolemeis. You have to understand, that Kings or Pharaos did not have just one wife, and so they had several children with several women, and they were not necessarily raised together, as there was a lot of powerplay by their respective mothers. And to have boys and girls raised in different ways is still a very real thing, and has been so in all of recorded history - and to send your children away to be educated has been almost as common in the elites. Thus, I find nothing strange in siblings being raised apart in the royal Egyptian or Greek family.