The Four Sisters, also called The Four Riders, are four, eternal, supernatural entities. Their powers are immeasurable as their age. Each Sister represents a permanent in the universe, and as such, they are truly immortal.
Amber is the youngest of the 4, and represents hunger and desire.
Amelia is the second youngest of the 4, and she represents sickness and suffering.
Araseli is the second oldest of the 4, and represents war and anger.
Ana is the oldest of the 4, and the oldest and most powerful being to ever exist. She represents life and death.
The Four Sisters are obviously represenations of The Four Horsemen of The Apocalypse, with Amber, Amelia, Araseli, and Ana, serving as Famine, Pestilence, War, and Death, respectively.