You don't, simple as that. You could say cousins if it's that important, or "my aunt's nieces and nephews on the other side," but there isn't an actual term for that except your "uncle's wife's siblings children."
Your sister's husband is your children's uncle. They are his nieces and nephews.
Your brother-in-law's children are your nephews and nieces.
Your brother's children would be your nieces and nephews.
He is their Uncle and they are nieces and nephews.
Second cousins.
Nephews are the sons of your brother or sister. Therefore nephews who are not brothers to each other are first cousins to each other.
Your sister's husband is your brother-in-law. Your brother-in-law's brother is not related to you and neither are his children. But he is your sister's brother-in-law and his children are your sister's nieces and nephews.
Nephews and/or Nieces
Nieces and nephews are the children of your siblings.
Children of your siblings are nieces and nephews. Children of your spouse's siblings are still called nieces and nephews, but you are not related to them by blood.
Your nephew's children are your great nieces (girls) and great nephews (boys). You are their great uncle or great aunt, depending on your sex.
Your great nephew's children are your great great nieces and great great nephews. You are their great great aunt or great great uncle.