Forbid Lore

Phantom is the elder of the siblings, their age and terrible health conditions had stretched thin their silk, fraying it and making it grey

Phantom heavily resents their mother, and see themselves abandoned and discarded; but there's nothing they really can do besides continuing the task she gave them (to block the easier entrance to the Citadel via producing the Mist) and wishing for death in combat

Throughout Act 1, and the first half of Act 2, Lace is utterly loyal and loving of her mother, in a blind fashion, and continues to do the task she gave them; Caretaker dialogue implying she's been doing this for a long while

Yet GMS radio-silences Lace despite all of her efforts, and in late Act 2, Lace comes to despise her own existence, feeling discarded and abandoned, eventually resenting her mother and, when given the possibility, denying her her revenge in the Snared Silk ending. Something very similar to what Phantom already feels since long ago, and since they're the elder of the siblings, they had more time to set in those thoughts

This implies that Phantom was also once just like Lace, and they got radio-silenced by their mother despite all their efforts to do the task she birthed them for. Lace's description states that a lot of silk was needed to sustain her, which coupled with Hornet mentioning that other silk lifeforms are transient and ephemeral, could explain why the age difference between the two siblings: GMS spent a lot of her silk in producing Phantom, and needed time to regenerate it before producing Lace

The First Sinner is, no doubt, one of the most beloved Silksong bosses, not only for her fight or her design, but also because of the amazing plot twist that comes with her defeat.

On the memory scene after her defeat, we see no Weavers, only feral Pharlids scuttling away on what will later become the Cradle; until one is reached by Grand Mother Silk's thread and is forcibly evolved into becoming a Weaver. Considering how similar the mask of that Pharlid is to the head of the First Sinner, we can assume that what we see is her herself being evolved from Pharlid into Weaver.

"She called us daughters... Called us divine... She lied..."

But if this was a great revelation, then the Weavers, including herself, were oblivious to their feral origins.

Now, how did she find out? The answer is easy: Silk. One of the many properties of Silk is that it can be used to look at the past through it, such being the case of the various Silk Hearts, or the defeat of Widow and First Sinner. Her journal entry reads:

Ancient Weaver condemned for a transgression lost to time.

This one's husk held a masterful talent, to weave Silk into blazing runes. Throughout my life, I have seen runes woven for many tasks. Was she the first to hone such skills?

This speaks of her having an unbelievable amount of ability for working with Silk, which goes in line with she using it to look into the past. Through masterful usage of her Silk, she looked into her own past, was witness to her origin, and discovered the truth of Weaver origins.

And for finding out about the Weavers not being divine, she was punished by her fellows for the sin of "apostasy", which is described as "the abandonment or renunciation of a religious or political belief or principle".

She was locked away on the Slab, constricted and suppressed, and given the inscription of her cell, she was the first one sent there, setting precedent for the Citadel sending further "sinners".

Penitent, First of the First.

Guilty of the sin of apostasy.

Penance by constriction.

Absolution denied.

There is more to her, though.

Before the First Sinner found the truth, Weavers saw themselves as divine, and were worshipped by the common bug as such, with various Weaver Effigies being carved as talismans of worship. In addition, various Weavenests were named after particularly important Weavers.

One such named Weaver is Atla, who is the only named Weaver mentioned twice in the entire game, and the Weavenest named after her is the most important one, for it not only holds Eva, but is also the only Weavenest to be its own, unique area; instead of a sub-area. It's almost like Team Cherry was telling us "Hey, Atla is important".

Near First Sinner's cell, there is a dumped Weaver Effigy. What is doing a talisman of worship like that in the Slab? Its description reads:

Carving of Atla, Weaver of time. A prayer is etched upon it.

"By grace of your example, and our history held full in mind, our song is yours, as your name is sacred. Weaver Atla, may you be praised eternal."

So, we have a Weaver Effigy that mentions Atla again, describing her as a "Weaver of time", dumped near the cell of a Weaver who read the past time through Silk and found out the truth. Furthermore, the First Sinner pulls Hornet into a Memory Battle to fight her and test her, she is very much manipulating time for it.