Personally, I wouldn't be stressing about it. You need to give the responsibility to the person who has the coeliac disease, or her mother, to make sure that she provides you with a suggested list off basic food items they would normally eat. They are only there for 3 days, so find out what she normally eats for breakfast, which could be the same each day. You might go out for the odd lunch/dinner, so only need to cook 1 or 2 of each. I'd keep it simple, opting for something vegetarian that can be cooked in a casserole, or baking dish, and frozen (there's some great vegetarian mediterranean/Spanish recipes on Pinterest that are my go-to when I have vegetarian guests). Serve with a nice salad, or mixed vegetables dish and, if you feel the need to provide a little extra for the carnivores, simply roast a chicken with herbs/spices and cut it up, or quickly pan fry some nice steaks.
And, definitely tell your husband, if he wants the visitors, he needs to do his fair share in the kitchen and around the house! Again, keep it simple, don't bother ironing the bed linen, and make sure you leave the bathroom spray by the shower and instruct your guests to give the sink/shower a once over when they've used it! They are family, so you can tell them to do that! ... And the Domestos by the loo!! You're not running a B&B, and you shouldn't have to work twice as hard just because you have guests staying.