Available Histamine Blockers for Hypertension Sufferers

My mother-in-law in Florida is medicated for hypertension, but is suffering has severe airborne allergies from different allergens.  Antihistamines normally prescribed to sufferers interact with her high blood pressure medication.  In addition to her heart specialist, she has seen two allergists and an ears, nose and throat specialist.  She has prolonged coughing attacks and averages 3-4 hours sleep/night - much of it interrupted.  It is making her miserable and the specialists she have visited are now literally taking guesses as to what can be done.  It has gotten to the point that she has occasional sinus infections.  While antibiotics alleviate these infections, the sinus problems remain, even when put on steroids.  What medications or treatments can be recommended to help guide the efforts of these specialists?  I find it incredibly difficult to believe she is the only hypertension patient in the United States that has this particular problem.