In the books
In the A Song of Ice and Fire novels the islands are called Sweetsister, Longsister and Littlesister. The islands are ruled by the houses Borrell, Longthorpe and Torrent respectively, although the archipelago as a whole is ruled by House Sunderland, based at Sisterton on Sweetsister. People from the Sisters are known as 'Sistermen'. 'Sister's Stew' is a noted dish popular on the islands.
The islands are noted for their roughness. They are popular with smugglers, traders and sailors, but are considered to be dens of avarice and sin by septons. In the distant past the islands exchanged hands several times between the Kings in the North and the Kings of the Vale, until the Vale achieved supremacy some two thousand years ago.
- Three Sisters at A Wiki of Ice and Fire (spoilers from the books)