You are their aunt or uncle and they are your niece or nephew.
They would be great nieces or great nephews.
He is your second cousin once removed. His children are your third cousins.
cousin
brother-in-law
Great neice
sisters
Sisters
Your sisters daygter is your neice!
Second cousins.
You would be their great Aunt.
They are sisters.
Your great grandparents brothers and sisters are your great uncles and great aunts.