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.
brother-in-law
cousin
Great neice
sisters
Your sisters daygter is your neice!
Sisters
Second cousins.
They are sisters.
You would be their great Aunt.