Major spoilers
There is much debate as to whether Susie was Mother Suspiriorum all along, became her at some point or emerged from within her.
However, what I find more perplexing is the role of Susie's mother in her deathbed in the film. We know from Kemper's account of Patricia's testimony that the three Mothers predate God and the Devil, so Mother Suspiriorum existed before Susie's mother.
Now it calls into question exactly what is the relation between all three characters. For example, we could have the following scenarios:
- Mother Suspiriorum at some point possessed Susie in childhood to fully emerge later on.
- Mother Suspiriorum is similar to a deity, and Susie's mother birthed her earthly form (similar concept to Christ, so Susie's mother was chosen by Mother Suspiriorum).
- Susie and her mother were a normal human family, and it is only through the events of the film that Susie is chosen by Mother Suspiriorum, given her natural talent described by Madame Blanc.
There are even more possible scenarios. My point though is, if Mother Suspiriorum existed before Susie's mother, then what is Susie's mother to Mother Suspiriorum?
Most online article try to address what happens to Susie and don't really mention her human mother at all in the film. I am guessing there may be clues in the dreams that were sent to Susie since these did feature her mother.