Learn Spanish (Latin America): Es la hermana, no la amiga.

In Spanish, nouns have a gender (masculine or feminine). Words ending in -a, like hermana (sister) and amiga (female friend), are usually feminine. You must use the feminine word for "the", which is la, so that it matches the noun.

If the sentence was about a brother and a male friend, how would it change?

You would change the feminine -a endings to the masculine -o endings, and change the article la to the masculine article el. The sentence would become: Es el hermano, no el amigo.

Where is the word for "she" in this sentence?

The word for "she" is ella, but it is left out here! In Spanish, you can often drop words like "I", "he", or "she" because the verb (es) already tells us who is doing the action. The feminine words hermana and amiga also make it obvious we are talking about a female.

Can no mean "not" as well as "no"?

Yes! In English, we use "no" and "not" in slightly different ways, but in Spanish, the word no covers both. Here, no la amiga simply translates to "not the friend".