Some sort of auto-immune disorder if I were to hazard a guess.
Tourettes?
I have MS, and I have a lesion in my brainstem...weird symptoms can be caused by lesions in the brainstem, especially autonomic functions; breathing, heart rate, bp, vomiting, hiccups,sneezing, etc.
I’m not suggesting she has MS, just saying that would seem a logical place to look and see if anything were going on in that area of the brain.
I just googled and sneezing is triggered by the 5th cranial nerve...if I were a doctor, which I’m not, I’d be looking at an MRI of her brainstem.