This is an unclear and rarely used phrase - "your brother's daughter through marriage."
If your brother married a woman and they had a daughter, the daughter is "your brother's daughter through marriage" since the one followed the other. That daughter is your niece.
If your brother married a woman who already had a daughter from a previous relationship, that daughter is your niece only if your brother adopted her. If he did not adopt her, you are not related. However, you can treat her as your niece if you want to.
Are you asking what you call your brothers daughter? Well if you are your brothers daughter is called your niece.
Through marriage your step - sister's daughter is your step-niece!
Since it's her husband's niece the niece is just your sister's niece through marriage and not your niece.
A niece
A niece
Niece
Your sister's daughter is your niece and you are her aunt (if you are a woman) or her uncle (if you are a man).
Your brother's daughter would be your niece.
Your brother's daughter is your niece..and you are her AUNT.
Your brothers ( or sisters ) daughter is your ( niece )
your step niece,,, if it was your brothers own daughter it would be your niece, since its his stepdaughter, than its your step niece.
Your niece.