Under the Yggdrasil Tree

My older sister, who is only a year older than me and lives next door, isn't my real sister, but she's apparently been taking care of me since I was born. It seems she has a secret wish: to live in another world. I got dragged into that wish. Furthermore, there are dangers everywhere in that other world... why would she bring that up? And where did she even get that information?! Well..., I guess it can't be helped if it's another world, and I'm just going with the flow. For now, my sister and I have to somehow manage to live in this world... This is the story of a boy who starts living in another world with that kind of resolve.