mmswm, I think it's important to remember that just because someone asks you a question, you do not have to answer it. I know your first instinct is to be nice and nice girls always do what other people want them to do.
However, "nice" is not the same as polite. You can refuse to answer a question. You just have to make a polite refusal.
My nephew was born with a rare syndrome. Part of his treatment for the first few years was wearing a brace from his hips to the top of his head. Of course, strangers commented on it. On good days, my sister-in-law would give a very brief account--"The doctors are worried about his spine, so the brace is protecting it." On bad days, she would say, "Well, we aren't letting his dad change his diapers anymore," and walk away.
And sometimes, as with relatives who pry too much and are too judgmental, a stare of disbelief is all you need.
I will admit that there have been times that I've questioned and doubted, to myself, how my brother and SIl handle my nephew's disabilities. But I have never, ever said anything to them that wasn't supportive. Because who the heck am I to question what they do? I'm not the one living 24/7 with that child and his issues and the very real threat that he could die very young. I see my job as his aunt as being supportive of his parents, and helping out as much as I can, both with him and his younger siblings--but that help is directed by his parents, not by what I might think.
The relatives who question you are rude. You do not have to give rude people any more information than you feel comfortable with. Etiquette requires that you refuse them politely. It does not require you to listen to people who are rude and mean to you.