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.
Your aunt's sister is either your mother or another aunt. Therefore, her children are your cousins. If her sister is your mother, then those children are your siblings. If she is another aunt, then they remain your cousins.
cousin
Great neice
brother-in-law
sisters
Second cousins.
Sisters
Your sisters daygter is your neice!
You would be their great Aunt.
They are sisters.