The Ptolemies did not observe the restrictions of the mother's side, but Ptolemies were not Egyptians. The cosmological ideas of ancient Egypt were expressed by myth and symbolism, which are a superior means for expressing metaphysical concepts. Philosophy, mathematics and science are dry subjects with many abstracts. Mythology incorporates philosophical and scientific ideas into a story form, like a sacred drama or mystery play, which can easily be digested. Pure philosophical and scientific abstracts and terms do not guarantee understanding.
Information alone is useless, unless it is transformed into understanding.
Well-crafted mythology can achieve that understanding. Isis and Osiris, like Amun, Ptah, Horus, etc., were not gods and goddesses. In the Osiris myth, Isis and Osiris, sister and brother, got married. The relationship between Isis and Osiris was purely an allegorical fable. Within a larger cosmology, it can be viewed as a type of divine love.