After dealing with the sirens, use witcher senes and examine the nearby dead body. Use Keira's lamp to examine the area by the body. You will see the brothers mourning the loss of the bride. Examine the area again with witcher senses to see footprints leading away from the body. Follow the prints, and it will lead towards a tree. The body of the brothers will be by the tree, they killed themselves by the tree, it turns out they would rather die than live with the shame. Loot the corpse to get a letter and a key.
Go back to where you find a bride's body and you'll see a sunken ship mast in the sea. Swim there and dive to open a chest with a necklace.
Return to the quest giver, and tell him what happend to his betrothed and his brothers. You can eiter give him the neckace of leave it.