Maybe you should just sit down and talk to them like they are adults. Find info on food aversion and sensory overload and share it with them. Then, take them to a fancy grocery store, give them some money, and help them pick out things that are "upgraded" versions of what they are comfortable with.....perhaps some gourmet crackers, a nice baguette, some high quality butter. Maybe that would spark some interest in food? Also, get them in the kitchen cooking. They don't have to eat what they make....make sure they feel secure that they don't HAVE to eat it. But just get them involved in the process. Tell them what you enjoy eating, have them help you look up recipes, shop for ingredients, and prepare it. Watch YouTube videos of proper chef techniques. Perhaps the exposure to preparing a meal would slowly lessen the sensory overload? Also, please explain to them about nutrients, why it's important to eat a balanced diet, and how harmful a limited diet like that is on their bodies.