Food allergies are a real pain in the a.. I have always been allergic to a few things , corn, chocolate, wild rice. Easy to avoid those things. Now corn is in almost everything and I have developed a sensitivity to MSG. Only it is rarely called MSG anymore, it is called by a dozen other names. I feel for your relatives.
She changed the family's diet for one week and the kids' behaviors and attitudes and general crankiness totally changed. They begged her to back on real food after that ... Which resulted in a relapse of the old behavior.
She's researched it a bit since then, but everything they have now has no additives or preservatives. Costs a little more and it makes shopping a treasure hunt. Or the other hand, I was surprised when she brought Steak Ums burgers to my house for a BBQ, but the package listed beef as the only ingredient. I would've thought that Steak Ums would be junk. Who knew?
Well, she did. Me, I got more to learn.
I feel for you. I am horribly allergic to msg and can feel it in the second it enters my mouth if I make a mistake. I have to avoid so many prepared foods. People are shocked to hear that it is in nearly every cracker and chip, and in EVERY SINGLE PREPARED SOUP period unless you are in a very serious gourmet restaurant who makes their own chicken broth. There is no chicken broth purchasable that does not contain added msg.
However, making your own is easy. After roasting a chicken and carving, throw the carcass into the crockpot with a little vinegar and veggies (onions, leeks, carrots,,etc) and a ton of water and the next day you have plenty of broth for the freezer.