The Sisters are considered the weakest of all the faction, only having three fiefs and seven lords. Their troop tree has some of the least infantry further disadvantaging them.
The Sisters were once ruled over by Local Pirate Kings. Similar to the Ironborn they worshiped their own religion outside of the Old Gods. After the Andals invaded this was put to an end. Sister pirates often times raided the North and in response The Starks invaded the Three Sister. In the hope of keeping the Northmen out the Sistermen bent the knee to the Arryns of the Vale. This led to a Thousand years of war between the Starks and Arryns over control of the Island. War ended when the Starks lost interest in the Islands. Even today Arryn control over the island is loose at best.