As other unborns (how we like to call them), they just want to be you. After that long time, the Siamese Twins keep wondering why they are not alive, why their soul is here, what happened.
As many, they don't understand, sometimes they are lost, sometimes upset, sometimes scared, and sometimes they feel attacked, as they revive the day when their way into our existence was turned into the wrong direction.
So then, they will protect each other. In this case, one has become a parasite soul, and will enjoy this madness, when the sorrowed soul lose the temper to protect her evil sister... who is the only one who can now accompany and love her forever...