Your sister is her great grandaunt.
Your brothers great granddaughter is your great great niece.
Your granddaughter is your sister's great niece. Your sister is your granddaughter's great aunt.
Your sister is the great-aunt (also written great aunt) of your granddaughter.
Your uncle's great granddaughter is your first cousin, twice removed.
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).
great aunt
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 aunt is great great aunt to your son's daughter (your granddaughter). Your granddaughter is your aunt's great great niece.
great uncle
Your brother's daughter is your niece and his granddaughter is your great niece.
Your grandfather's great great granddaughter could be:your granddaughteryour great niecefirst cousin, twice removed
Your grandfather's brother is your great uncle. Your great uncle's daughter's daughter (his granddaughter) is your second cousin.