I know the whole Gwyn is related to the Vanserras may seem outlandish but hear me out and thank you @shesnothere9 pointing it out.
There was no reason for Sarah to connect Gwyn to the Autumn Court which we know will be part of the overarching plot but she did because:
Gwyn has Autumn Court features;
“Beron—slender-faced and brown-haired—” Beron's hair is brown and his face is described as slender.
“The fox-masked faerie sank onto the edge of the table, the light catching in his long fire-red hair.” < Lucien's hair which means Lady of the Autumn Court has Fire-red hair.
“Eris lounged in a golden chair, legs crossed, his pale face the portrait of courtly arrogance.”
“Eris's long red hair[...]”
“Eris reached out a long, slender hand[...]”
“A pale, beautiful face appeared above her,” < Eris.
“The two males were similar-looking, in the way that people from individual courts tended to share characteristics: the Autumn skewed toward hair of varying shades of red, brown or gold eyes—sometimes green, and mostly pale skin.” < the two males here also had hair one that was auburn but more brown and the other bright copper.
“Her hood was thrown back, and faelight danced in the rich coppery chestnut of her pin-straight hair. Her large teal eyes were as clear and depthless as the stone usually atop a preistess's hood, and a scattering of freckles lay across her nose and cheeks, as if someone had tossed them with a careless hand. She was young—almost coltish, with her slender, elegant limbs. High Fae, and yet ... Nesta couldn't explain the way she sensed that there was something else mixed into her. Some secret beneath that pretty face.”
“Gwyn's coppery-brown hair appeared. In the sunlight, the color of her hair was extraordinary, strands of gold glinting, and her teal eyes were a near-perfect match to the stones the other priestesses wore.”
“My grandmother was a river-nymph who seduced a High Fae male from the Autumn Court. So I'm a quarter nymph, but it's enough for this.” Gwyn gestured to her large eyes—blue so clear it could have been the shallow sea—and her lithe body.
Meanwhile, her fraternal twin Catrin had webbed fingers of the nymphs, black onyx hair and pale skin as the moon. It's not clear whether these features come from the father or the mother since it seems nymph blood is more dominant in Catrin.
And Gwyn's family history from that shared makes me suspect their connection to the Vanserras;
“My mother was unwanted by either of their people. She could not dwell in the rivers of the Spring Court, but was too untamed to endure the confinement of the forest house of Autumn. So she was given in her childhood to the temple at Sangravah.”
And we know her father is a mystery because when her mother came of age she took part in the Great Rite and then became pregnant with the twins. She never found out who the male was and no one came to ask about the twin girls.
What's interesting here is that it was not mentioned that her mother was too untamed for the Autumn Court, the Autumn Court is vast but what was specified is the Forest House.
Beron’s seat was so vast it took us three days to enter, pass through, and clear it. Lucien led us through the outskirts, tense at every call and rustle. The Forest House was a sprawling complex, Lucien informed me during the few times we risked or bothered to speak to each other. [...] No enemies approached Beron’s home without his knowledge.
Her mother as a child was living in the Forest House before she was sent to Sangravah. Why would she be there in the first place? There are the farmlands and other lands in the Autumn Court meanwhile the Forest House is Beron's home. Surprising especially if Gwyn's mother is half High Fae and half lesser Fae and we all know how Beron feels about lesser Fae.
I'm speculating that either Beron or Eris are the ones related to Gwyn, but the more I think of it the more I think Beron is the grandfather. I did speculate before about Eris because of the “not as far as the first born” line and the fact that he did not take part in the death of Jesminda.
How ironic it would be if Beron was indeed seduced by a lesser Fae and he had a child from that River nymph. It could've been a one time thingS The story may even be darker than that, Gwyn's grandmother realizing that her child can't live in the rivers being half High Fae so she brings her to the Autumn Court but Beron kills her (Gwyn's grandmother) and the child (Gwyn's mother) is saved because of Eris because she is still his baby sister but they can't keep her in the Forest House long because Beron wants her gone and as the text says, she was sent to the temple to Sangravah.. So that could make Eris and Lucien possibly Gwyn's uncles.
It's all speculation but it's interesting to find these connections in regards to Gwyn's family because it feels like SJM was specific about some details and the Autumn Court is part of the main plot. I mean come on it's the Autumn Court, out of all the Courts they're the ones with the family drama.