Hold up I cant follow... Im lost Waaaiiiitt Does this story has regression tag? Or isekai?...

ehh i’mkinda confused too i just started reading again from where i left off (forgot most things so don’t think my word on this that seriously , it seems like his past memories with his little sister IS real but that little sister was wrote a novel with her friend (that other girl with them) and then an accident happened which most likely killed them all ORR they went into a coma(this would make kinda more sense since he claims he’s in a novel and etc) and then they were put into said novel that the sister and her friend wrote ?? idk.. that’s what i’ve gathered so far …