It depends what type she is, and what she normally eats. If she’s type 1 she may eat exactly the same as you, she just want to know carb content for calculating her medication needs from each meal. (Keeping packaging handy for her to look at may help with this)
If she’s type 2 it depends on what she normally eats. If you prepare carb free meals and tell her she can’t eat this and that when she normally eats everything she lays her eyes on. Your going to fall out with your mother in law very quickly. Ask her to give you some suggestions of what she has for breakfast, lunch and dinner, meals she likes and anything she avoids. Then you can think it over and maybe run a few of your regular meals past her see if it’s something she would try, or how she would adapt it.