Learn Spanish (Latin America): El abuelo es bajo, pero el hermano es alto. - The grandfather is short, but the brother is tall.

Because they describe el abuelo (the grandfather) and el hermano (the brother). Both are masculine, singular nouns, so the adjectives describing them must also use their masculine, singular forms (bajo and alto).

How would this sentence change if we were talking about a grandmother and a sister?

It would become: La abuela es baja, pero la hermana es alta. You have to change the articles (el to la), the nouns (abuelo/hermano to abuela/hermana), and the adjectives (bajo/alto to baja/alta) so that everything matches in the feminine form.

The lesson says adjectives usually go right after the noun. Why are bajo and alto separated from the nouns they describe?

When you use the verb es (is) to describe someone, the adjective comes after the verb, just like in English (for example, "the grandfather is short"). The "after the noun" rule applies when you want to group them together into a single phrase, like saying "the short grandfather" (el abuelo bajo).

Why do we use es here instead of está for the word "is"?