
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.
the art is getting worse. I don't know much about art but this is basically a sketch with colors.
I think it's fine. This is the artist signature art, it actually have an ancient aesthetic to it, not like the other common generated art around that basically is no different than other manhwa
I didn't say the artist's signature art is bad. I meant to say quality dropped is quite noticeable in the last few chapters.