I used to sneeze alot after I took showers. Not sure why, but it rarely happens now. My diet has changed and I also take much quicker showers and use only mild fragrence free soaps and shampoos. Eventhough you've switched brands, they still probably have many common ingredients. Good Luck!
My dad has asthma and cannot handle high humidity. Maybe the coughing could be from that??? Does she cough after her showers, too???Best wishes with you and your mother-in-law.