Lol I’m pretty sure that the humidity isn’t aggravating her sinuses/allergies. Rather, it’s other particles in the air that are doing so (unless she is allergic to water).
While acknowledging that increased humidity will affect how she experiences symptoms, it is important to understand the distinction in order to properly address the root of the problem. Your proposed solution of a quality air purifier should solve your problem! Meanwhile, the dehumidifier shows that you understand and take her experiences and opinion seriously. Good work on your part!
I have an air purifier at home, not because we need it, but because I was already intimately familiar with the product, and had an opportunity to grab a NIB one for an unbelievable discount by a manager that was leaving the company. Mine actually increases humidity, but is nonetheless certified by the Asthma and Allergy Foundation... regardless of certifications, it works noticeably well. But look around, find an air purifier that works well for y’all.