Learn Spanish (Latin America): Mi tercer hermano es muy alto.

Tercero drops its final -o and becomes tercer when it comes directly before a masculine singular noun. Since hermano (brother) is a masculine singular word, we use tercer. The only other ordinal number that does this is primero (first), which becomes primer.

What if I wanted to say "my third sister"? Would I still use tercer?

No, you would say mi tercera hermana. Ordinal numbers act like adjectives, so their endings must match the gender of the noun they describe. Because hermana is feminine, you use tercera. The rule about dropping the final letter only applies to masculine nouns.

Why does tercer go before hermano, but alto goes after the verb?

In Spanish, most descriptive adjectives like alto (tall) go after the noun they describe. However, ordinal numbers like tercer (third) are an exception. They give a specific order or rank, so they almost always go before the noun, just like they do in English.

Why use es for "is" in this sentence instead of está?

Es comes from the verb ser, which is used to talk about permanent traits or core characteristics. Being tall (alto) is a physical characteristic of the brother, not a temporary state or location, so es is the correct verb to use.