Your brother or sister's daughter is called 'your niece.'
Your sister's daughter is your niece.
Your sister's daughter is your niece. You are her aunt (if you are a woman) or her uncle ( if you are a man).
niece
Cousin.
Your father's sister is your aunt. Your aunt's daughter is your first cousin. Your first cousin's daughter is your first cousin, once removed.
its called a niece
1st Cousin
Your sister's daughter is your niece. If you are a woman, you are your niece's aunt; if you are a man, you are your niece's uncle. Your sister's son is your nephew.
he has two. his older sister is called lynne and his younger sister is called joanne.
If the brother-in-law's daughter is also your sister's daughter, she would be a niece. Otherwise, there is no relationship.
I know of no Egyptian goddess called Kemse-Mies, but Bast was daughter of Ra and any other daughter of Ra could rightfully be called her sister.