"Esta muchacha es la hermana de Juan."
This translates to:
"This girl is Juan's sister."
Here's why the original sentence was incorrect:
* "El" is the masculine singular definite article, while "la" is the feminine singular definite article. "Hermana" is feminine, so "la" is the correct article to use.
* "El hermana" is grammatically incorrect, as it's trying to use the masculine article with a feminine noun.
Therefore, the correct translation is "This girl is Juan's sister."