I think First Materials cannot be directly affected by pymary. This magic uses aspects to directly influence materials, like adding heat or removing solidity. That can't be done to First Materials, but it can be influenced by its effects.
I speculate that Sette is a model for the works of the khert. The senet beasts we saw so far are made of first material, later "models" like humans and animals are subject to the khert. That is a pretty massive change, and so far we saw for pretty much everything some kind of model; for basic movement, for mountains, for metamorphosis, for gender dimorphism, for sexuality. Sette is subject to khert, and Duane confirmed she scans as human a few chapter ago. But she doesn't age; Anadyne says she didn't age in 3 years so she is not subject to decay like regular humans.