Why do Salicylates cause pain?

My older sister has arthritis and for the past four months she has only eaten foods that are Salicylate-free. Everyone has a threshold of tolerance to chemicals in food and her threshold must be lower than mine because she reacts to them and I do not seem to. A build up of Salicylates over a week or so causes her arthritis to flare up. Her body can not deal with the build up of chemicals in her body. Mine can.