idk why it was cut off last chapter but its missing context for the sudden mention of his sister so I shall fill in the context. at first, after being told he has to debut with spArk, he declines. the system tells him that if he debuts, his sister can come back. he's in shock by that because his sister died years ago (to him). when he tries to call her, the number isn't recognised. when he tries to leave to find her himself, the system gives him a pop-up and he can't remember where she lives (or what her name is iirc..). then it follows into what's at the start of this chapter, with his abusive parents and everything else.
this might not be 100% accurate, since i read the novel a while ago now, but at most i missed minor details or put them in the wrong order.