Point out to TW, calmly, that an alcoholic, narcissistic, bipolar, reality-warping machine part is no less an unnatural monster than we are, and that it's to our mutual benefit as monsters to stick together, but if it comes down to a separation, she needs us more than we need her - she has no friends nor allies, and no ability to gain them. Tell her that from a strategic standpoint, her unique abilities and insights make her an asset, but her attitude and behaviour make her a big liability -- and from a personal standpoint, make it very hard to like her or consider her a friend.
Remind her that there are really only three ways things can work out for her.
- She plays nice, sticks with us, and gets to have a hand in building (and shaping) the next configuration of the world as we know it.
- She leaves, drinks way too much with nobody to keep her in check, and either (a) falls into a stupor and is recaptured, or (b) goes on a destructive drunken rampage until Small Horse and/or the military bring out the really big guns and obliterate her bony ass, magic or not.
- She fights us. If we win, we get a spare banshee in case the original is destroyed. If she wins, see Option 2.
It's her life and her choice. Maybe she'd better take some responsibility for it, instead of assuming Small Horse or Zero will take care of it for her all the time like the parent of a self-absorbed teenager.