I has no problems getting through the quest, as I'd finished the Thieve's Guild questline at a high level and/or got lucky enough to get the stronger verions of the Amulet of Articulation, so Sivdi spilled her guts right away. I did lie to Sibbi, but who wouldn't--he's such a self-entitled, leering jackass. (My character is female and his dialogue is VERY gender specific).
I got the key, went to the chest, and it was completely empty. Ungrateful bastard. Now I'm really glad I sent him chasing to Morrowind.