Sorry you're in this situation, Kartrush! All that worry, plus it hurts to feel as if you (general) are the "only one" left out of the loop. Please try to remember this is not about you - it's about DIL, who may/may not be ill and her right to privacy, and DS, who, after all, has his wife's concerns to think about.
Unfortunately, DIL seems to have made a big mistake in whom she did choose to tell (your niece and, perhaps, some of her own family) if she wanted to protect her privacy. Same if she and DS are just trying to protect you from worry (which may very well be), as some have suggested.
But, at this point, I agree, you would be wise to say something. I'm chiming in with those who say to approach DS - and to just say you've "heard rumors" and ask if they're true. Then, please, accept whatever he says, for now, and, above all, don't discuss it with anyone else. Please respect their privacy, even if others don't.
Wishing the best for DIL...