as a historian, let me expand on that. To say that it's an issue of "blood purity" is a bit simplistic, it might be better to say that it was done to "preserve power" and to preserve the image of nobility. It's argued that the rationale behind incenstuous unions in royal or noble families is that kings were supposed to be "like gods"--that is, like literal gods walking on Earth. So, it was no big deal if they married and had children with their own blood, because the gods did it. While incestuous unions might have been prohibited for the common people (either by secular law or by religion), nobility was the exception to these kinds of rules because marriage within the family preserved the (political) power of that family.
At the same time: Ancient Egypt wasn't a monolith, and incestuous marriage was not a given among the nobility, let alone a given between first degree relatives (siblings).