Three Sisters

The Three Sisters' are a region in Game of Thrones. They are a group of islands in the Bite. They are part of the Vale of Arryn, the region under the control of House Arryn of the Eyrie. They are the northenmost part of the Vale and lie close to the port of White Harbor to the North.

In the books
In the Song of Ice and Fire novels the Three Sisters are a group of three islands Sweetsister, Longsister, and Littlesister. They are considered dens of avarice and sin by septons. Men from the Three Sisters are known as "Sistermen." They are ruled by House Sunderland.