I really think your frustration here is just because MIL is an unbelievably tactless individual. She has an opinion on everything. This latest is just the straw that's breaking the camel's back.
It might have been different if she had asked 'what if DC gets the disease? Would you need help to cope with it?' etc. And try and express her concern differently..
What's the attachment like between DH and her? Has she ever let go? Why is she taking such an unhealthy degree of interest in his relationship with you? I'm quite shocked. Whatever happened to children growing up, maturing and being entitled to some bloody privacy?! It's OK not to answer her, btw. It's okay to say 'that's a v personal question, don't you think?'
Of course this is her grandchild, but I really think your DH should be intervening more to curb the worst of her excessive and intrusive opinions. She's had her turn, it's yours now.
Fair enough if she has concerns about her grandchild, but there are ways to express these that show support for you all, and other ways that are just likely to get your back up.
If I were you, I'd get your DH to take the worst of the flack. Control the frequency and length of their invasive visits. (Shame, but it may be the only way of maintaining civility with her unless she changes her ways.) And visit them sometimes - that way, you can choose when to leave.
I'm sure you're smart enough to make your parenting decisions jointly with DH, with your son as the no.1 priority, rather than making decisions to spite MIL. Then DH can defend your decisions if he wants to.