I know that I sound like an idiot but I did NOT understand the last chapter at all!!!!!! ...

well i think the brother do something that made his sister do something extreme and had a split personality , and the sister delusionaly think his brother were inside her and forget about the real brother existence , this plot really similiar with the chapter 5 who the wife have mentally ill and forget about his husband , the different in this story is that in the chap 6 they're incest while the chap 5 the actually husband-wife (๑•ㅂ•)و✧

Thanks for the explanation