I have a slightly different twist on your in laws, they are your children's grand parents, they are clearly unwell and you are very dear to them. You might want a quick brief contact even if only by phone, wishing them well, or send a small note via the kids, just saying that you want the best for them. It would be a small kindness for your kids.
Agree with V here, I don't see the big deal about being nice to the inlaws if you had a good relationships before. I'm quite cozy with my inlaws, I like them, they like me, and I think it's important for my kids to see that their grandparents are worth spending time with, even though their dad left me. He's the odd man out, not me.
