Are the daughter of my father’s first wife and I mahrams?

Assalamualaykum… i am the only son of my father’s second wife…first wife of my father has passed away, through her he has one girl and one boy… 1)is the daughter of my father’s first wife non-mahram for me ? 2)is it necessary for her(the daughter of my father’s first wife) to do pardah from me as normally done with non-mahrams? 3)Are we purely brother and sister and mahram for each other? Jazakallahu khaira wa ahsan al-jaza