I always liked to think that she actually had Cassandra's Tears, which drove her to instability. Lydia's introduction in the same book was a little too coincidental to me, and Lydia wasn't exactly a picture of mental health. Her association with a different Vampire Court also appealed to my sense of symmetry.
It kind of ties in to my thought that those with Cassandra's Tears are descendants of second-born children of the Archive maternal line.
I don't know if she's a changeling or not, but I'd be interested if she had some kind of connection to the Fomor.