Thieves in Sigil are dominated by two figures: the arcanaloth Shemeshka and her rival, the ursinal Ni'ka. With these powerful personalities ruling Sigil's underworld most have little chance to do anything without the go-ahead of one or the other.
To quote Dave King's Dreams of Dreams website:
Still, despite this reputation, Ni'ka is still a good creature at heart. She's fooled most everyone in Sigil as to her true motivations, and they see her more as "one of them" than they do as a servant of Elysium, which serves her aims quite well. Her extortion and larceny are evils, of course, but she makes sure that only the most deserving receive her treatment. She even manages to funnel some of the goods and cash that she steals into charitable works, something like a Sigilian crusader for equality and freedom. 'Course, she'd never let anyone know this, and keeps those that work for her completely in the dark regarding information.
There are few jackal lords in the Cage, as most prefer the easier task of dominating small towns with few supernatural rivals, and they dislike having the only canines available to mimic Sigil's mongrel packs of wild dogs. Still, the City of Doors is a powerful temptation, its portals bringing wealth and power from a thousand thousand planes, and some consider the challenge worth it. For them, the presence of Shemeshka and Ni'ka is more of a benefit than a hinderance, though they would feign admit it. Left alone, they would likely spend more time trying to rid themselves of one another than accomplishing anything useful, but the overlords of crime often exert their formidable clout to persuade them into other schemes.
One jackal lord, Queen Bis, is currently one of the three most trusted servants of Ni'ka (the other two are a vermin lord known as the Queen of Things That Creep and an unseelie thorn fey assassin named Honeysuckle). Queen Bis is served by naztharune rakshasas, witchknives, changelings, humans, and protean scourges. She lauds her access to the King of Thorns before her rival jackal lords at every opportunity, even intimating that they are lovers (they are not, but Ni'ka does not contradict her). Like all of Ni'ka's primary servitors, she also runs a thieves' guild.
Another jackal lord, Bishop Jah (Prime/male jackal lord cleric of Set/LE), is served by jackalweres, shifters, human cultists of Set, and flinds. He is ultimately commanded by the Marauder, but he is several levels down from any position of authority in her organization. He chafes at the rule of his immediate superior, the examiner (beholder abomination) wizard Khuxuwes, and openly envies Queen Bes. He also receives commands from the Minions of Set, and must delegate frequently to accomplish all of his responsibilities and still have time to scheme to bring Queen Bes down. His most trusted underlings are both human clerics of Set.
The jackal lord known as the Go-Between serves neither Shemeshka nor Ni'ka exclusively, but attempts to play each off of the other. The Go-Between is a guildmaster of sorts for the city's jackal lords, working to advance their interests in exchange for regular gifts. In reality the Go-Between is more Ni'ka's than Shemeshka's, but for this to become widely known would ruin him. He is also visited infrequently by Duke Dyavin B'wee, an arcanaloth ally of A'kin's who serves to further widen the gap between the Go-Between and the King of the Cross-Trade. The Go-Between keeps no servants, but is surrounded by both human and jackal concubines.
Ni'ka. (Blackheart, King of Thorns). Elysium/Female ursinal mystic theurge/NG
Honeysuckle is worshipped by Sigil's bladelings as an avatar of their goddess Hriste. She is served by bladelings and needlefolk.
Shemeshka has her own bladeling assassin, Adamok Ebon, who left her race millennia ago. Ebon has acquired several parties of astral stalkers who she dominates as if she were an exceptional member of their own race. They hunt for her when she is otherwise engaged.
The Queen of Things That Creep is served by xixchil, thri-kreen, humans, rogue formians, and abeils, but her most prized servants are simple vermin. Her sentient servants she mostly ignores, and they are in most practical situations led by the abeil queen Lady Alustriel (no relation). They venture into the den of the Queen of Things That Creep only when they are absolutely forced to, but she is too powerful for Lady Alustriel to usurp; at least, so far.
An interesting view on the criminal underworld of Sigil (and a good way to use some antagonists that would otherwise be neglected). I'll eventually come up with my view of the underworld and some of the players involved...
I certainly see post faction war Sigil as being quite the struggle between various organizations, criminal or not.