It is difficult to see Melie suffer like that, i don't doubt that Venelope too must have suffered a lot too, loving her sister but having to acknowledge that her sister was "crazy".
This "yandere" personality is really throwing me off.
Usually, it has the character doing the whole "mine forever/anything for you" thing, which this one is.
Oddly though, this "anything for you" seems like it's actually that. It's an obsessive compulsion, just like regular yandere, but it feels totally fresh (to me at least) in execution.
by the title of this chapter im guessing this personality is "happiness/elation" since what she was feeling before the switch was deep sadness and despair....