
Am I missing something? What's the point of his sister being there in Shae's body? Is she possessing Shae's body even relevant to the story?

It's kinda relevant actually, otherwise Shae will never alive in the first place. Maythan twisted the whole plot of the story so everyone's happy and Shae's brother is reunited with his sis at last.
Also she played a big part to bring Maythan back to real life, helped him knock a sense out of him to not forget the real objective.
so the chapters are actually shorter and it isn't just my imagination.