Thinking you can pass the buck to an incarnation? No way.
The God King isn't oblivious to the principles of incarnation. This silly sister might show off her cleverness in front of others, but to do so in front of her is just asking for humiliation.
Although the original magic was indeed created by her sister, the manuscript she left behind became the starting point for today's witch magic. But to be fair, if you give that manuscript to mortals now, even mortal mages would find it outdated.
If you ask who truly elevated witch magic to its peak, it could only have been the God King herself.
Denisha did it out of nostalgia for her sister, using her sister's manuscript as a basic framework, wanting to fulfill her sister's unfulfilled wishes.
