I’m in the Demon Cult, So Why Is the Empress Teaching Me to Stage a Coup?

When Jin Chang'an woke up, he discovered that he had become a member of the Demon Cult. Just as he was feeling lost and helpless, he stumbled upon a ring. To his surprise, the remnant soul of a big sister was living inside it. So this is the ring grandpa trope, except it’s big sister? With the big sister’s guidance, he soon rose rapidly through the ranks and became a high-ranking member of the Demon Cult. But just as the cult leader was leading him to assassinate the emperor, he suddenly realized something. Wait, how did the big sister in my ring become the reigning empress?