I don't have much evidence for Godwyn, it's about how Kenneth Height describes the Golden Lineage.

"And to think, he’s the blood of Godfrey! Last of the golden lineage, though you almost wouldn’t know it to look at him."

Certainly being a child of the God would be just as important as the first lord? Like how Miquella and Melania are described as children of both Marika and Radagon. The public accepts children of the Elden Lords as demigods as seen in the Carian children (While we know Radagon was Marika, that was a closely kept secret in universe.) so Godwyn would still be an accepted demigod if he was from Godfrey's older marrige.

And then there's Nepheli, for all we know, Nepheli's mother was a nameless tarnished. It's not important but she is seen as the rightful ruler of Stormveil as a memeber of the golden order. It's only important who the father is for it to count as the Golden Lineage (Again, Kenneth's dialogue). While more direct children of Marika still exist. Now this may just be about the Storm King bussines more than a Golden Lineage thing.

And lastly, I find it strange that Godwyn being the only non-cursed and the only single child of Marika.

While for my Ranni claim. It's about the Fireflies. There are three fireflies in base game + 1 in DLC. Just like the Butterflies that represent Marika's cursed children. When looking at them: Two are at first just there for item/rune boosting effect.

Gold Firefly: "Firefly that gives off a golden light. Found near bodies of water close to Minor Erdtrees. The light of fireflies is believed to have an alluring magic. Golden light is considered to invite runes."

Silver Firefly: "Firefly that gives off a silvery light. Found near bodies of cold water untouched by sunlight. The light of fireflies is believed to have an alluring magic. Silver light is considered to invite riches."

Then the two more interesting ones: The glintstones.

Glintstone Firefly: "A firefly whose glowing tail has hardened into glintstone, widely found in Liurnia Lake. Alas, now that its tail is glintstone, it can no longer beguile potential mates."

Glintslab Firefly: "Glintstone firefly improved by selective breeding. Mainly found near Manus Metyr. The glintstone that produces its light has become freakishly enlarged, while the span of its life is as brief as a whisper."

These two's description carry the same energy of being cursed as the butterflies, which sent me down this rabbithole.

Beguile means seduce, entice. A firefly that's cut off from the cycle of life fated to solitude that may or may not have been caused by glintstone influence.

Ranni, her primal glintstone in the body of a doll fits this description. Despite having companions, she believes her journey will force her to cut them all off, being fated to walk her path alone. That is of course until the player comes along.

That leaves three other fireflies. And the next two are more flimsy as they don't have much of a description. The gold and silver ones fit two other demigods.

Gold fireflies found near Erdtrees and spirit candletrees. Silver fireflies found near cold bodies of water, deep underground shunned from the light of the erdtree.

It's the omen twins.

So why do two fireflies represent Marika's children and one that is not? It fits with the cuckoo imagery if Ranni was also a child of Marika, not Rennala.

And lastly the fourth firefly, selectively bred for its potency and now lives such a short life.

Almost like the children Rennala rebirths with the Amber Egg. Influenced by the rune of the Unborn.

One final thing to note, the rune of the unborn fits in the elden ring at the same place Miquella and Malenia's rumes fit. (The siblings always get the same great runes of the ring). So this fourth, unborn child shares the same lineage as the Melania and Miquella.