The Sistermen originally worshiped the Lady of the Waves and the Lord of the Skies instead of the old gods adopted by the First Men of the mainland. They would also cast dwarfs into the sea as an offering to the gods, but septons stopped the practice after the coming of the Andals.
According to a semi-canon source, Sistermen can wear sealskin cloaks.
The Sistermen were once a free people whose rulers acted as pirate kings. In response to the Rape of the Three Sisters, the Sistermen bent the knee to the Eyrie to expel the invading northmen, ending the era of native kings. The Kings of Mountain and Vale from House Arryn and Kings in the North from House Stark fought over the Three Sisters for a thousand years in the War Across the Water, depleting much of the islands' resources. Some Sistermen still resent northmen.