My interpretation for how Gwyndolin came to be is that he is a replacement for Filianore, Gwyn's youngest daughter. It's strongly suggested that Filianore was actually the first fire keeper, but she was terrified by a premonition she had of the failure of Gwyn's plans and the fading of the First Flame. Gwyn removed her eyes and stuck her in the Ringed City to guard the Dark Soul, and lied that he would return for her one day. To replace her, he sired Gwyndolin with Priscilla, the half-breed bastard abomination, and possibly had assistance from Seath to make a fire keeper out their child, judging by Gwyndolin's association with sorcery, the moon, the Moonlight Butterfly, and the Pisacas, which were evidently once fire keepers themselves, and why Gwyndolin was raised a daughter. It's possible that beneath his helmet, Gwyndolin also lacks eyes, just like the rest of the fire keepers in DS3, so as to prevent him from realizing the inevitable rise of the dark.
From descriptions of the fire keepers' souls, a fire keeper receives the humanity that is offered to the fire for kindling and is gnawed by their attraction to the fire. The more humanity she harnesses, the stronger the fire burns. This is also true for the Chaos Flame and the First Flame, and without a fire keeper, dark humanity would escape into the world again, and the flame would dissipate.
However, when the Age of Dark encroached upon the world, Gwyn relieved Gwyndolin of his duty as the fire keeper, and offered up his own soul to embolden the First Flame. Gwyndolin was relegated to guard the now desolate city of Anor Londo, and charged with convincing a "Chosen Undead" to follow in Gwyn's footsteps and become the fuel for the First Flame, at the cost of their very soul. (The Anor Londo fire keeper mentions that Gwyndolin "made her" a fire keeper, perhaps by giving up his own fire keeper's soul, which he no longer needed.)
If you look at Anor Londo from the side, you'll notice that beyond Gwynevere's chamber is another staircase leading to a solitary tower. This tower is the exact same design as the one where we find Princess Filianore in the Ringed City, indicating that she once had a role in Anor Londo, but was moved to the Ringed City, a sort of imitation of Anor Londo, much later. (The missing statue in Ornstein and Smough's arena could have depicted either the Nameless King or Filianore, both of which were meant to be forgotten for their failures.)
As for what happened to Priscilla, she was once kept imprisoned somewhere within Anor Londo, but when the world became unhinged after Gwyn sacrificed himself, her soul found peace in the Painted World of Ariamis. The Painting Guardians were stationed to prevent anyone's access to the Painted World, and to imprison whatever inhabitants sought to escape. They exist to hide the secret of Gwyn's tryst with Priscilla, perhaps even from Gwyndolin, her own bastard child.
When Gwyndolin falls ill in DS3, he becomes prey for Aldrich, who is assisted by Sulyvahn, a native of the Painted World who discovered the truth about the gods and sought to destroy them. Yorshka is possibly the daughter of Priscilla and some unknown entity, perhaps one of the two painters, Ariamis or Ariandel. It is possible that Gwyndolin truly is the half-brother of Yorshka, and the discovery of such a fact is what prevents Gwyndolin from assisting the Chosen Undead any further, as his ability to serve his father's will is destroyed by his subsequent disgust. He is rendered defenseless, and Aldrich has no trouble gaining access to Anor Londo and consuming Gwyndolin.
Anyway I'm crying again so my keyboard might be broken in a seco