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 (though the exact location of House Sunderland's castle-seat is not clear). 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. The Three Sisters were too small to resist outside conquest and become a regional power in their own right, and thus are regarded as mere pirate dens. In contrast, the Iron Islands raided surrounding lands by sea but were larger and strong enough to be an independent kingdom, so their "piracy" was enshrined as a cultural institution, "paying the iron price".
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. During the War of Conquest, the Sistermen seized on the opportunity of the invasion to rebel against their overlords from House Arryn to side with the Targaryens.
- Three Sisters at A Wiki of Ice and Fire (spoilers from the books)