Yeah I suspected this from the beginning but it sure means that the older siblings fucked up royally the first time around. Like how do you go from "we'll do everything to protect her" and then let her be ostracised, let her believe you all hate her, and let everyone else believe it too. To the point that she becomes an easy enough target of harrasment that she gets framed for murder!
Like they did a shit fucking job the first time and they're lucky that their baby sister is using her head in ways a normal baby/toddler/teen would not have been