Jane is only the name of her cover. Her true self is a succubus called Janith.
That said, she hasn't returned to her true self or even just used her true name in a long time, because Janith have fallen in love with her prey. And why would she not? William love her back, is handsome, never deals in a disrespectful manner with anyone and, cherry on top, is wonderful on bed. He also happen to be a great father to their childs, but she was unable to check that before their birth. (That and at the time, just suggesting that she will have a child with an human, to her, looked like a joke.)
But she never revealed to him her true nature. Can you imagine how ridicule that will sound? "Willy, my love, I'm not really a woman. In fact I'm a succubus, and my true name is Janith. But my species and names were my only lies to you. I really love you! I promise, I have, and never will, feed on your life-force! I love you to much for that! So please, don't reject me…" Like that got any chance of working… He will flee as soon as she say "I'm a succubus" if she don't enthrall him before… And she will die sooner than enthralling William.
The worst in that, is that she haven't eaten the life force of any one, not since William and her are a couple. For some reason she don't know, their love seemed to feed her with even more sustenance that the souls and lust of her victims was ever able to.
If only she was able to make him listen to what she have to say until the end of it, NOT by making a thrall out of him…
Alas, you can't hide a secret that huge to you family forever. And if someone else reveal it before you do, the result might be an ugly one, as Jane soon discovered…