Here's a corrected version and its meaning:
"C'est la fille de mon frère" This means: "She's my brother's daughter."
Here's a breakdown of why the original phrase is incorrect:
* "Ses": This is the plural possessive adjective for "his/her/its." It's not used for a single person.
* "La gosse": This is a very informal, almost slang term for "girl" and is not generally used in polite conversation.
* "Mon frere": This is correct, meaning "my brother."
More formal alternatives:
* "C'est sa fille." (She is his daughter.)
* "C'est la nièce de mon frère." (She is my brother's niece.)
Informal alternatives:
* "C'est la fille de mon frère." (She's my brother's daughter.)
* "C'est sa fille." (She's his daughter.)
* "C'est sa petite." (She's his little girl.)
Choose the appropriate phrase based on the context and your relationship with the listener.