a subling is usually a small person (unless your old), that is the offspring of your sibling.
'sublings' refers to the group of people we currently call "nieces and nephews". Since we use the word 'siblings' to refer to our "sisters and brothers", we need a related word for their kids. Just replace the 'i' with a 'u'.
Since I have lots of siblings and they have lots of kids, I have an army of sublings. I buy birthday cards in bulk.