A long unfinished poem by Coleridge that some argue to be an inspiration for Le Fanu's Carmilla, which I have just read. And yes, there is that shared central relationship between girls, both mirroring each other, and the fascination between them resting on Geraldine's monstrous nature just as it does with Carmilla's. What is also reminds me of, though, is Stoker's The Lair of the White Worm, because Geraldine is presented with a very serpentine imagery. If she's not a snake in a woman's skin I'd be frankly astonished, but then Coleridge only completed the first two sections of a planned five, as I understand it, and it's not like he's coming back to finish it off. Given that, I'm sticking with snake, and my guess is that Christabel's dopey enamoured dad was going to get bitten and come to a nasty, venomous end.