The rules relating to intestacy are different to when a will has been left.
If your brother had left a will, leaving his estate equally to his siblings, then the children of a sibling who died before him would NOT inherit. (The inheritance only 'moves down the chain' when the named beneficiary is a child or grandchild of the deceased, not when he or she is a sibling).
However, in the case of intestacy, anything which would otherwise have passed to the (now deceased sibling) of the person whose estate is being distributed now passes, in equal shares, to their children.
So, yes, your nieces/nephews (who were the offspring of your deceased siblings) ARE entitled to receive their parents' shares of your brother's estate.