How likely is it that chronic sinusitis results in serious complications such as vision loss, meningitis, aneurysm/blood clot?

Pretty rare, BUT: Lots of folks have chronic sinus problems, but most of the time, localized misery over nasal areas, and occasional postnasal drip with secondary coughing. If an acute infection supervened and was not treated, this could eventuate in secondary complications such as cavernous sinus thrombosis, but such complications are very rare. Suggest you see otolaryngologist to assess treatment.