Could someone tell me what they know of the Raven Queen's origins?
Citing sources is a plus.
For a D&D reference I'll point you to the related Wikipedia article (it bears a good summary of the Raven's Queen back history).
For a real-world mythological reference, this post goes to a fairly good level of analysis on Greek and Norse mythologies.
Manual of the Planes has a large number of references and on page 60 give some details on her realm.
Open Graves: Secrets of the Undead also gives a lot of information scattered thorugh out it's pages. Page 22-23 has a whole section write up on her.