First thing you need to find out is, what type of diabetes is it? People with Type I diabetes (juvenile diabetes) have to take insulin daily because their bodies don't produce it anymore. Type II is tied to insulin resistance...which can be brought on by many different factors.
My youngest sister has Type I. She manages it by watching the amount of carbohydrates that she eats in a given day. This means she doesn't regularly eat extremely sugary foods...but when she does, she remembers to take more insulin.
I'm sure your to-be mother-in-law would tell you that it's something she manages...not the people around her. You could end up making a big deal out of something that isn't really a big deal to her. Unless you bought the jumbo economy sized favor boxes, a small amount of sweets shouldn't be a big deal. She may not even like pillow mints or conversation hearts in the first place

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If you have your heart set on being accommodating...you can do some favors that mix in raisins, peanuts, and other dry fruits.
Diabetic Snacks