nece and nefu i spelled it wrong though sorry if someone could update this better Your daughters are his nieces by marriage and your sons are his nephews by marriage.
Your sister's husband is your children's uncle. They are his nieces and nephews.
Your kids and my sister's husband bear no relationship to each other. It is extremely unlikely that they even know each other.
He would also be your cousin.
He would be your brother-in-law.