If they want to comment, then I would just simply ask, "then why is it that your kid (or SIL's child, if speaking to grandparent) is always sick and mine isn't?"
At least Grandma is very on board with our choices. I mentioned this exchange to her (I guess sil had also talked to her because she knew about it before I said anything) and she just kind of shrugged like "what can you do". I did also warn her (in general) about the risk of a shedding vaccine. She is immunocompromised and I would hate to see her get sick.