Down Among The Sticks And Bones by Seanan McGuire
This dark fantasy novella delves into the backstory of Jack and Jill, twin sisters who, discontent with their strict upbringing, stumble through a portal into a grim otherworld. In this realm, they find themselves on divergent paths: one sister apprentices under a mad scientist, embracing logic and science, while the other is swept into the world of a vampire, seduced by the promise of power and belonging. As they navigate this eerie and dangerous world, their bond is tested, and they confront the harsh realities of choice, consequence, and the deeply ingrained desire to find a place where they truly fit in.