You would be the child's(?) uncle or aunt.
cousin
brother-in-law
Great neice
Your sister is great aunt to your daughter's child. The child is your sisters great nephew (if a boy) or great niece (if a girl).
Your sisters daygter is your neice!
sisters
Sisters
Second cousins.
You would be their great Aunt.
They are sisters.
Your great grandparents brothers and sisters are your great uncles and great aunts.
No relation