Learn Spanish (Latin America): Mi hermano es moreno y guapo. - My brother is dark-haired and handsome.

It can mean dark-haired or brunette, but in Latin America, it is also very commonly used to describe someone with a dark or tanned skin complexion. The exact meaning depends on the context!

Just as a refresher, why do we use es here instead of está?

We use es (from the verb ser) because we are describing inherent physical traits. Even though looks can change over time, being dark-haired (moreno) and handsome (guapo) are treated as core characteristics of a person in Spanish, which always require ser.

If I wanted to say my sister is "extremely dark-haired," how does the -ísimo ending work for females?

Adjectives with this suffix still need to match the gender of the person. You would take the feminine base word morena, drop the final -a, and add -ísima. So you would say: Mi hermana es morenísima.