Sisterton is a castle in the Vale. It is located on the middle island of the Three Sisters, a three-island archipelago located in the Bite, which are considered part of the Vale, thus falling under the authority of House Arryn.
In the books
In the A Song of Ice and Fire novels, Sisterton is located in the same point. It is the chief port of the Three Sisters and serves as the seat of House Sunderland. It is also considered the most notorious smuggler's den in all of Westeros.
- Sisterton at A Wiki of Ice and Fire (spoilers from the books)