I was a candle

This sounds like a great opportunity to bust out my terrible MS Paint skills.

Okay so there’s a lot going on here and not all of it is directly related to the question but I thought I’d put it in anyway.

So, anyway, let’s go.The Bazaar and the Sun have a child, the Mountain of Light, but the Sun is actually in (unrequited) love with some other star. The Bazaar is in love with the Sun, but the Sun isn’t really in love with it. The Bazaar was sent with a troth to the other star from the Sun, but got a “No” in response. The Bazaar is hiding out in the Neath to try and postpone delivering that rejection notice, because there’s a very real chance that the Sun will die of sadness and extinguish if it hears that it got turned down. Please stop saying that the Bazaar is trying to prove that love between different links on the Great Chain is possible to the entire host of the Judgements, or you’ll make me sad. The Mountain of Light is referred to as “the Sun’s experiment” in one of the Destinies, which is probably important.

The Bazaar’s grand-daughter, then, is Mt. Nomad, the giant spooky obsidian blob at the bottom of the tree. Mt. Nomad was created by the Thief of Faces, here represented by a picture of a face. The Thief was chilling out in the Garden with its other creations, the Snuffers, when it decided to steal some jewels to make Mt. Nomad with. For this, the Thief was locked in the Prison of Flint, and the Snuffers were cast out of the Garden. The Thief got out, though, and it’s been raising hell ever since; it uses Mt. Nomad as an operating base now and then. The Thief changes faces and forms constantly, working to destroy what it was and hating what it is. It loathes absolutely everything, including the things that brought it into the world; the Flukes.

The Flukes, Lorn or otherwise, come from a far planet called Axile. They’re the progenitors of the Rubbery Men, as well as the Thief of Faces. Generally, “Flukes” refers to the weak small Flukes in Flute Street who struggle to survive, while “Lorn-Flukes” refers to the huge strong buff ones who thrive in the peligin depths of the zee. 

The Fathomking married one of the Lorn-Flukes, and in doing so became a weird incredibly powerful fish dude with a tremendous store of secrets. The Drownies are related; you become a Drownie by eating fluke-lumps (and then dying, presumably), or by being “baptised in the bride’s cup”.

The Fathomking is technically tied into the Bazaar’s family tree through the Flukes, who are tied through the Thief of Faces, who is tied to the Mountain. From what we know right now, this is the only feasible link I can come up with.

Unrelated matters on the chart that I put on anyway: Humanity is said to have been cast out of the Garden, along with the Snuffers, though possibly not simultaneously. Humanity doesn’t remember this, while the Cousins do. It’s entirely possible that humanity was born under the Mountain, or maybe we just broke in once. Maybe that seems obvious if you consider the Garden to be Literally The Garden of Eden, but it’s one of the wombs of a half-star half-spacecrab in the form of a giant glowing diamond mountain, so, y’know. I suppose it is worth mentioning that the Wistful Deviless in Sunless Sea says that devils said up shop in Parabola back when “men were monkeys still”, so presumably humanity didn’t just appear overnight, at the very least.

Anyway, human souls return to the Sun and are consumed upon death (being in the Neath doesn’t seem to change this; blame the Boatman, who is almost certainly working for the Judgements in some way). You could consider this to be like some great circle of life, or a wheel forever-turning.

Final note on the question mark: The Bazaar is said to have other children, according to Slivvy, the prophetic urchin-wizard who speaks with Storm. I really can’t find much to fit with that, but he’s presumably not making stuff up, since you get that nugget of information in response to selling him the secret of the Nadir and all. Since some urchins are actually part giant eel on their father’s side, perhaps it’s not all that far-fetched that some of the others aren’t entirely human either.

IMPORTANT FAMILY TREE CHART KEY:
Sun clipart: the Sun
other sun clipart: other sun
Bazaar spire: Bazaar
Mountain of Light: Mountain of Light
Face: Thief of Faces
Snuffer: Snuffers/Cousins
Rubbery Man: Rubbery Men
Fluke: Flukes, especially the Fathomking’s Bride
Fathomking: Fathomking
Drownie: Drownies
Wineglass dude: All Of Humanity
Question mark: ???