If she's unappreciative little brat, it's fine to comment on it, but why do you need to mention whether or not she's related by blood? IMO that's no different than saying "we buy my niece presents and she's unappreciative, and she's gay and we still care about her!"
But she's NOT my niece. My nephew is my nephew because he's my brother's offspring. My brother s not married to my nephew's mother. My nephew's mother is not my sister. My nephew's mother is married to some other random man. Her and this random man had a child. This child is not my niece. She is my nephew's half sister.