A 'bts twin replace fanfic' is likely a work of fan fiction related to the BTS group. Fanfics often take creative liberties. In this case, the idea of a twin replacing a member could lead to many different plotlines. For example, it could be a story about a twin who has been hidden away until now and suddenly replaces a BTS member for some secret or mysterious reason. It might also be a more light - hearted story where the twin just fills in for a day and all kinds of comical situations occur as the other members try to adjust to the new 'twin - replacement' situation.
I'm not entirely sure specifically what a 'bts twin replace fanfic' is without reading it. But generally, in the BTS fandom, a fanfic might involve creating a fictional story where a twin character replaces one of the BTS members in some sort of scenario. It could be a really creative and unique take on the group's dynamic, perhaps exploring how the other members would react or how the story would unfold with this new element of a twin character in the mix.