My sister is allergic to nuts and when she was younger her friends used to try to trick her into eating nuts. And there are so many parents that are totally unsympathetic to the idea of peanut free schools. Your mother in law is cut from the same cloth. Deep down she thinks you are a drama queen and that your food issues aren't that big a deal.
Your mistake is engaging her. YOUR HUSBAND NEEDS TO BE THE POINT MAN FOR YOUR FAMILY'S RELATIONSHIP WITH YOUR MOTHER IN LAW. HE NEEDS STEP IN, SIT YOUR MOTHER IN LAW DOWN, AND HAVE A SERIOUS TALK WITH HER. He can start by describing in graphic detail what your IBS attacks do to you. Then he can tell her that she MUST take your food issues seriously because IBS is a serious condition, he loves you, and she should respect you.
You taking on your mother in law is like a step mother trying to be the main disciplinarian with her husband's kids. Your husband should be taking the lead.
Been through it~!
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Adopt the pace of nature: her secret is patience. ~ Ralph Waldo Emerson