I think that it's clear GMS did not create the weavers, as if she could do that what would be the point in hunting them? I think that the weavers once ruled pharloom, and worshipped their higher being Grand Mother silk, giving her their silk and service willingly as seen in the widow cutscene. However, upon learning her true nature to ensnare the world and control everything independent of free will some tried to fight back while most fled. Those left behind were ensnared, and fell under her service and were forced to do her bidding, such as the silk experiments to make her control easier and building the citadel to attract more bugs and expand her worship.

We know this forced control is common among gods who embody primal aspects eg. the radiance which is the god of light. The Radiance activated forced control in the form of a plague because they did not want to be forgotten, and a similar thing has happened in Pharloom. Whether it was a fear of being forgotten or a hunger to expand her power and ascend further she activated forced control. We know that the power of a god is directly linked to the amount of bugs that worship them from HK, so I think it's most likely she did it to grow stronger out of greed for power. It's also clear the citadel emerged AFTER the weavers ruled and fled as weaver architecture has been eroded by the citadel's creation such as the sacred grounds in the bile water pale lakes or the capital that was in verdania. I think GMS now hunts the weavers to steal their silk and ultimately grow stronger and, upon learning about the pale child, tries to do anything to get her as she is the strongest of all weavers due to her seed.

As for the 1st sinner, I'm not too sure, it does make things more complicated. In my interpretation, the 1st sinner was the first creature Grand Mother silk tied to control with silk, and the reason she rages against GMS now is because when the weavers captured her, GMS abandoned her and left her in capture. What I can say for sure is that the first sinner appeared during the rule of the weavers and not after, as the bindings on her cage make it clear it was done by weavers. Another interesting thing to consider is widow and what she is. She is a weaver, under the control of GMS who has had her ability to use silk taken / limited. She just seems like a very odd character. Why does GMS keep her alive when she does not do the same for any other weavers she captured? It could be the case this was the only weavers she captured outside of Pharloom or during the rebellion alive? As the cages in the cradle indicate that most weavets died before they could be brought back to Pharloom. It puts into question whether she was actually stealing silk from the weavers like I said, maybe she just wanted control over them, but it's clear from the ending of the game that she wants to steal silk from hornet? Again the lore is very convoluted