The Castle of the Two Sisters is a building in the Everfree Forest introduced in the Everfree Forest update. It is not an actual building per se as it is implemented as a background image. Currently, there is no way to interact with it although it is mentioned in dialogue as being full of secrets, so it may in future.
To access it you need to remove a boulder as part of a quest allowing you to claim the expansion it is in. Next to it is a ingredient called Black Iris, a dark purple flower used to create totems. The Castle of the Two Sisters is mentioned in a quest to defeat Lord Tirek in front of the castle.
The castle appears in the top of the Everfree Forest map and as a icon on the overworld map for the Everfree Forest