You are not related to your best friend's brother-in-law's grandma's great niece, unless you are related to your best friend's brother-in-law, to his grandmother, or directly to the great neice.
The son or daughter of one of your siblings children is your great niece or great nephew. In other words, the grandson or granddaughter of your brother or sister is your great niece or great nephew.
Your brother's daughter is your niece, her daughter is your great niece and her granddaughter is your great great niece. It works the same for nephew, aunt, and uncle.
Your niece's children are your great nieces and great nephews, and you are their great uncle. Your uncle could be the father of your niece, and thus the grandfather of your niece's children.
You are a great great aunt or great great uncle to your great great niece (or great grand niece).
You and your grandmother's great niece are second cousins.
They are cousins.
Great niece's baby is your great-great niece or great-great nephew.The system works like this:Your niece's child is your great niece/nephewYour great niece's child is your great great niece/nephewYour great great niece's child is your great great great niece/nephewYour great great great niece's child is your great great great great niece/nephew...
Yes, your niece's daughter is your grand niece. You are her great aunt or great uncle.
Your husband's great niece is also your great-niece in law, so your great niece would be great niece in law to your husband.
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.
You are her great-aunt/uncle. She is your great-niece.
You are the great-great-aunt/uncle to your niece's granddaughter. This means that your niece's granddaughter is your great-great-niece. The term "great-great" signifies that there are two generations between you and your niece's granddaughter.
When a couple is married and the wife has nieces who are daughters of her brothers or sisters, those nieces are also the nieces of her husband. So your great grandmother's sister's husband could have great nieces who are related to you and also great nieces who are not related to you.Note that your great grandmother's sister is your great great aunt.The great nieces who are related to you are the grandchildren of the siblings of your great great aunt.Those could be your:parentauntfirst cousins, once removedsecond cousins, once removed
The granddaughter of your niece is your great great niece.
You are Great Aunt to your niece's child. The child is your great niece or great nephew.
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 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.