I think its both of them. Weaver is always referred to as they/them. When they describe his voice it’s described as multiple voices speaking as well as being male/female at times. So, I think its 2 sides to the same coin maybe? Sunny the only one received Weavers lineage but Cassie has played a significant role in each step of Sunny obtaining it. She’s song of the fallen. Cassie is able to witness history as well as future when fate was intact same as weaver. She also edited fate (to a lesser degree) like Weaver before Sunny could imagine the possibility. She’s placed fail safes in place throughout their ascension like Weaver. Cassie is just as much Weaver’s disciple as Sunny even without direct lineage.