However the original cause of her insanity isn't addressed, so you'd think that without the vamp sex, Justine would revert because she'd be like any other mentally ill person who reverts when they are off their meds..
It's not the sex itself that helps the condition, it's the
feeding. Sex is just bait and the channel the Raiths like to use to get the energy. They can feed in other ways, like Skavis and Malvora do. (IIRC, JB once said that in theory a WV could feed on the warm and fuzzy feeling from petting a kitten, though he added that the WV would probably sparkle if they did it, so he might have been joking about that).
The White Vampires feed on emotional energy, which is also life energy. Justine in her manic state generates too much positive energy, in the depressed state too much negative. The emotions are the energy and
vice versa. When Thomas feeds on Justine, it takes away those
excess energies, leaving her more or less 'normal'...as long as he doesn't take too much or too often, of course. In that respect, a manic depressive is the perfect prey for a WV with enough self control to limit their take, because they can be fed on without hurting them, and even to their benefit.
(Ignoring the potential issue of supersex addiction, of course.)
And yeah, once Thomas couldn't feed on her any more, one would expect that she would either revert over time to her manic/depressive state, or have to use medication to control it. Which might be where Nemesis got its hooks in.
