The Lover's Requital

Either bribe or persuade the guard at the door in the jail in order to see Sibbi if not already done so. In Riften jail speak to Sibbi Black-Briar after getting the information for the quest Promises to Keep.

Walkthrough

 * 1) Sibbi will tell you he wants Svidi dead. When pressed for details, he will explain that he was due to marry her but was also romantic with another woman. Once Svidi found out, she had her brother confront Sibbi and Sibbi killed her brother.
 * 2) Either agree to help him or say no. Don't worry about her now, she can be found in Ivarstead when going to High Hrothgar.
 * 3) She is the bard in Vilemyr Inn. Her name is Lynly Star-Sung, speak to her and say: "Your real name is Svidi isn't it?"
 * 4) Either intimidate or persuade her to tell the truth depending on level of skill in Speech. However, even with 100 Speech, attempts to persuade her will not work without the Amulet of Articulation.
 * 5) She will deny who she is and say that Sibbi is lying. She says that her brother did not have a weapon. Then, she will be afraid that the Dragonborn will tell Sibbi of her whereabouts.
 * 6) Go back to Riften jail and either tell Sibbi about her or lie to him.

Either way, the reward will be a key to a chest in the basement of the Black-Briar Meadery.

Bugs

 * After talking to Lynly Star-Sung, Sibbi Black-Briar might be found in the cell next to his original cell. Waiting a few hours (4) seems to fix this.
 * After you wait some hours when he is at the wrong cell and he will be back he might not be able to speak to you. Go to the last autosave and unlock the door if you are with the guild you can clear your bounty and if you aren`t you will be taken to outside the jail, but when you come in again the door is open, then go talk to Sibbi.
 * After talking to Lynly Star-Sung, Sibbi Black-Briar is sometimes sitting in the chair on the left-hand side of the cell bars, or sleeping, and the player might not be able to highlight and speak to him. This can be fixed by waiting outside the jail until Sibbi is walking around or by picking the lock of the cell door to enter the cell and talk to him.
 * After talking to Lynly Star-Sung and arriving at the Riften Jail at night, Sibbi may be found sleeping. However, if you "wait" in front of his cell until morning he may have got out of bed on the wrong side and be stuck inside the wall behind the hanging flag.
 * Even with 100 Speech, attempting to use persuasion on Lynly (Svidi) will never work . The intimidation option will be successful on the other hand, depending on your level. A solution to this would be to acquire and equip the Amulet of Articulation at the end of the Thieves Guild quest Under New Management. The persuasion dialogue should now be successful. (confirmed on )