I can't find this "crone" anywhere.
what do we know about her?
she is extremely old and powerful, and is extremely proficient in Shadow/Tree Magic, accordingly to chris avellone.
http://bootstrike.com/Torment/Online/ravelpuzzlewell%20.php
She has 3 signature spell: black-barbed shied, black-barbed curse and blacksphere.
Any idea on what her stats could be?
You could also just make her a wizard/druid/mystic theurge so that she isn't restricted in her spellcasting at all. That'd probably be truest to Chris Avellone's intentions.
She should be in her epic levels so that she can cast true dweomers. Ravel can do things, like sever a man's mortality, that no one else can do.
If you kill her you're only severing one branch. She still exists, her branches winding throughout the planes.
although I am not too keen about shadowcaster..
this is maximum level night hag. how to take it from here? remember, she has forgotten more magic than the Nameless one shall ever know 
Night Hag (Advanced Hit Dice) CR 13
Always Neutral Evil Medium Outsider (Evil, Extraplanar)
Init +1 (+1 dex)
AC 22 FF 21 Touch 11
(+1 Dex, +11 natural)
HD: 16
HP: 136 (16d8+64)
Fort +14 Ref +11 Will +12
Speed 20ft
Base Atk +16 Grp +20
Attack: Bite +20 2d6+6
Full Attack: Bite +20 2d6+6
Full Attack: Greatsword +20/+15/+10/+5 2d6+6 (19-20/x2)
Space 5 ft. (1 squares) Reach 5 ft. (1 squares)
Abilities Str 19(+4) Dex 12(+1) Con 18(+4) Int 11(+0) Wis 15(+2) Cha 12(+1)
Stat Points Gained From Advancement: 2
Total Feats: 6
Skill Points: 152
Skills: Bluff +16, Concentration +19, Diplomacy +16, Disguise +16, Intimidate +16, Listen +17, Ride +16, Sense Motive +17, Spellcraft +15, Spot +17
Disease(Ex): Fortitude save DC 22(+8 HD, +0 Racial, +4 Con, +0 Feat) avoids disease.
Demon fever - incubation period 1 day, damage 1d6 Con. Each day after the first, on a failed save, an afflicted creature must immediately succeed on another Fortitude save or take 1 point of Constitution drain.
Dream Haunting(Su): Night hags can visit the dreams of chaotic or evil individuals by using a special periapt known as a heartstone to become ethereal, then hovering over the creature. Once a hag invades someone's dreams, it rides on the victim's back until dawn. The sleeper suffers from tormenting dreams and takes 1 point of Constitution drain upon awakening. Only another ethereal being can stop these nocturnal intrusions, by confronting and defeating the night hag.
Change Shape(Su): A night hag can assume the form of any Small or Medium humanoid.
Spell Resistance(Su): 25
Damage Reduction(Su): 10/cold iron and magic
Immunity to fire, cold, charm, sleep and fear(Ex):
Detect chaos, evil, good, law, magic(Sp): At will (CL = HD of Night Hag)
Magic missile(Sp): At will (CL = HD of Night Hag)
Ray of Enfeeblement(Sp): A coruscating ray springs from your hand. You must succeed on a ranged touch attack to strike a target. The subject takes a penalty to Strength equal to 1d6+1 per two caster levels (maximum 1d6+5). The subject's Strength score cannot drop below 1. (CL = HD of Night Hag)
Sleep(Sp): Will save DC 12(+1 Spell Level, +1 Cha) null
(CL = HD of Night Hag)
Etherealness(Sp): At will as long as the Night has her Heartstone (CL = HD X 2 of Night Hag)
no insight at all?
Advanced night hag only has a few spell-like abilities. I suggest you replace them with full spellcaster levels, the same way a psionic mind flayer gets 9 levels of psion, night hag could have wizard 8 (or better illusionist: shadow if you're using Pathfinder).
My take would be CR 25 (Lothar's level of power)
NE night hag [wizard 8: illusion, shadow]/druid [shadow archetype or substitution levels made from 2nd edition Shadow Druid kit] 5/arcane hierophant 10.
what about going higher with levels than that?
Didn't she lose some of her power? I don't remember the game well.
You could add levels of shadowcrafter (Underdark) or even shadowcraft mage (Races of Stone). Personally I think that would be too much compared to other NPCs in Sigil. A similar problem is with high level mages from the Prime transferred to Planescape - most powerful factols should be above them. Maybe when Paizo releases the mythic rules there will be a workable way.
she said she has forgotten more magic than the nameless one will ever know. but is still formidable. she still is killed by TTO however, so that one is even more powerful.
The sleeper suffers from tormenting dreams and takes 1 point of Constitution drain upon awakening. Only another ethereal being can stop these nocturnal intrusions, by confronting and defeating the night hag
You could make her a shadowcaster (Tome of Magic) with druid as her base class and some levels of mystic theurge.
You could also just make her a wizard/druid/mystic theurge so that she isn't restricted in her spellcasting at all. That'd probably be truest to Chris Avellone's intentions.
She should be in her epic levels so that she can cast true dweomers. Ravel can do things, like sever a man's mortality, that no one else can do.
If you kill her you're only severing one branch. She still exists, her branches winding throughout the planes.