ma belle sœur et nièce (only my sister is beautiful)
mes belles sœur et nièce (they are both beautiful)
note that "belle-sœur" in French sounds exactly the same, and means sister-in-law. A French speaker hearing this will certainly think of your sister in law.
My niece called asking for money. My brother's or sister's daughter is my niece.
Nièce.
soeur is "sister" in French
To write "my sister" in French, you would say "ma soeur."
Tu es belle
de belles femmes
vous etes extremement beau
désolé ma chère soeur
écrivez Troglodyte mignon belle
"Ma soeur fait ses études au collège."
beautiful
Sister