Here is the dialogue for the quest:
(Definite Spoilers!)
Thorim: Speak, <class>.
Player: Can you tell me what became of Sif?
Thorim: For a moment I thought you were my beloved Sif... that strand of hair... but no! It can't be... Sif is dead....
My... my brother.... killed her!
Player: He did more than that, Thorim. He controls Ulduar now.
Thorim: Ulduar... I was sworn to protect Ulduar.
Sworn to protect the Terrace of the Makers.
Sworn to protect the pacts between the races of frost.
But I failed! I let my brother trick me. I let him turn me against my allies. He tricked me into blaming Sif's death on a dear friend, Arngrim, the king of the frost giants.
After I realized what I'd done... I... gave in to despair.
Player: It needn't end this way.
Thorim: Who are you, little one? To come speak these words to the mighty Thorim...
I should... crush you!
Yet, you've delivered me from my stupor. Your presence here is a sign!
It was no accident that I mistook you for Sif. What would she think of me? I must prepare for battle.
I must avenge her death and liberate the titans' sacred grounds from my brother's madness!
(Definite Spoilers End)
(Possible Spoilers)
Hmm, it appears that Lok'lira the Crone may be more than she appears.
Maybe him confusing you for Sif was mere illusion on her part, but maybe it was not.
(End Possible Spoilers)