Your granddaughter and your grandniece are second cousins or third cousins to each other
You granddaughter is a great great niece of your aunt. Your aunt is your granddaughter's great great aunt.
Your sister's daughter is your niece. Your sister's granddaughter is your great niece Your sister's granddaughter's daughter (your sister's great granddaughter) is your great great niece. You are her great great aunt, if you are a woman, or her great great uncle, if you are a man.
Great niece
If you are a woman, you are great great aunt to your niece's granddaughter. If you are a man, you are great great uncle to your niece's granddaughter. In either case, your niece's granddaughter is your great great niece.
Your great great niece is the granddaughter of your niece or nephew. Your daughter and your niece or nephew are first cousins. Your daughter and your great great niece are first cousins, twice removed.
your niece
Your granddaughter and grand niece are second cousins to each other.
Your brothers granddaughter is your grandniece.
Your niece's daughter is your great niece and her granddaughter is your great great niece. You are the granddaughter's her great great aunt or great great uncle.
Your niece's granddaughter is your great great niece. You are her great great aunt (if you are a woman) or her great great uncle (if you are a man).
Your sister's grand daughter is your grand niece or great niece. You are her great aunt or great uncle.
Your sister's son's daughter is your sister's granddaughter and your great niece. (Some would say "grand niece.") You are her great uncle or great aunt.