Complex sentence: Takeya's stepmother, and one of the few people who can keep the rambunctious Natsuki in line; she taught Natsuki all of the techniques that she uses to beat her big brother up with, or so it appears.
Split into simpler sentences: Takeya's stepmother and one of the few people who can keep the rambunctious Natsuki in line. She taught Natsuki all of the techniques that she uses to beat her big brother up with, or so it appears.