What do you want to do?
One option could be a letter. The tone of it would depend on the reasons you are no-contact.
If it's something unresolved and unresolveable that lies between you, you could express regret ... something like "Hallo Brother/Sister, Sarah has let me know of your illness. I know things have been difficult between us for a long time, but I was sorry to hear this. Whatever else, we shared a childhood. I wish you well.
Sincerely
@butter1fly "
It's a bit formal and empty perhaps, but it might be enough to bring your sibling's child comfort.
Or you could chat more informally and warmly, which might or might not lead to something more.
Either way I'd consider a letter for your neice/nephew's sake, if not your own.
I know that Mumsnet is highly unforgiving and in some situations, no contact should stay until the end. In others, a degree of softening might help you, and perhaps your sibling as well as the other family.
Again, in some situations no contact should stay that way until the end. But I suspect that in other cases, some people who have tried to reach out a bit sometimes feel better themselves - more healed - after twenty or thirty years than those who stay adamantly no contact.
It depends on the person and the reason for no contact and the amount of time that's passed.