The majority of patients with a diagnosis of any brain tumor, will experience at least one episode of a seizure.
I'm no brain surgeon, lol!, BUT the brain lining is NOT a protective barrier!
This means that any compression from the tumor is likely to affect the lining and not the skull, right?
Now should this tumor indeed cause compression on the adjacent neurons inside the lining , this alone would mean a possible interference on neuron communication which is via electrical signals. The end result could be something like an electrical short-circuit which manifests as a seizure.
Your sister needs a second opinion, to definitely rule out Brain Tumor Related Epilepsy (BTRE) and/or any other related pathology.
To my opinion, the coincidental explanation, should not be accepted too hastily, even if nothing notable was visible on the imaging. Subtle brain abnormalities which could be of diagnostic value, can be easily missed.
Ketogenic diets and treatment with CBD oil have shown a lot of promise in alternative therapeutic approaches to seizures.
I happen to know a few patients who have experienced remission of their seizures,
epilepsy and at least one person who had a Parkinson's dx.
Their conditions however, were not related to a brain tumor.
It's worth looking into it, while waiting for a second opinion (if she decides to go for that).
I hope this helps, but please note that my comments are not intended to replace medical advice.
Best wishes,
Niko