Your granddaughter is your sister's great niece. Your sister is your granddaughter's great aunt.
Your sister is her great grandaunt.
Your sister is the great-aunt (also written great aunt) of your granddaughter.
great aunt
Your granddaughter and your sister's grandson are second cousins.
nothing
Your sister's daughter and your daughter are first cousins to each other. Your sister's daughter's daughter (her granddaughter) is first cousin once removed to you daughter. Your granddaughter and your sister's granddaughter are second cousins to each other.
Your Grandmother's sister is your great aunt. Her grand daughter is your second cousin and her great granddaughter is your second cousin, once removed.
The half-sister of your granddaughter's half-sister is your granddaughter, or is not related to you.
Your grandson and your sister's grandson are second cousins to each other.
second cousin twice removed?
Your great aunt is the sister of your great grandparent. Her granddaughter is your second cousin and her great granddaughter is your second cousin, once removed.
Your child and your sister's daughter are first cousins. Your grand child and your sister's daughter are first cousins, once removed.