Sister of Light is a relic that can be found in The Darkness 2. It is a replica of part of the Metatron's Stele.
The Metatron's Stele was a stone slab carved directly from the Seat of Creation by Enoch, containing every revelation he witnessed in Heaven. Before the Stele got lost to time, someone managed to copy sections of the Stele, "The Sister of Light" being one of those parts. The "Sister of Light" in question is The Angelus, who was created when God separated Light and Dark, with the Light becoming The Angelus and the Darkness staying the Darkness.