Idk if I'm the only one with this theory but I'm gonna say it here. *Lord of Shadow spoilers*
Okay, so we know the Seelie Queen wants the Black Volume because she can use it to help get the Unseelie King out of power and she thinks its safest with her. But, I think that an underlying reason could be to raise Sebastian from the dead. I mean it would make sense because she appeared to have really liked him and the strong theory they have a kid together.
Another way to contribute to this is Clary's dream. If the Seelie Queen needs related blood like Malcolm needed Blackthorn blood, then the Queen would either turn to Jocelyn or Clary since they are the only living blood relatives.
And I have been told that the Seelie Queen would have been needing to preserve Sebastian's body for this to happen, and I understand that. But Annabel had been dead for a while before Malcolm figured out what really happened and she got her body. And only a couple of years have passed since Sebastian died so his body might still be in okay condition for this to happen.