Well, I would say "no" they really don't have "allergies". Excess dust, mold, etc. are irritating to their respiratory system and can cause infection or damage the lungs and air sacs.
Are her nostrils free of debris or food? I've never had Silkies, but some people find that they need to clear the nostrils frequently - seems their nostrils get clogged up. http://www.browneggblueegg.com/Article/PluggedNare/PluggedNare.html
Personally, I would not treat her with an antibiotic...
Does she sneeze all the time or do you notice it more when she has been eating/drinking?
Since she is almost 2yrs old and has sneezed since she was a chick and you don't mention that any of your other birds have ever had any problems, the likelihood that she has a respiratory disease is small...