Learn Polish: W porównaniu z bratem ona jest wysoka. - Compared with her brother she is tall.

The phrase uses the preposition z (with), which requires the instrumental case when it means "together with" or, as used here, "compared with". The dictionary form is brat (brother), and its instrumental form is bratem.

What case is porównaniu and why?

It is in the locative case. The root noun is porównanie (comparison). The preposition w (in), when expressing a state or abstract location, requires the locative case. Neuter nouns ending in -anie change to -aniu in this case.

Could I say od brata instead of z bratem here?

Not with the phrase w porównaniu (in comparison), which strictly pairs with z + instrumental. However, if you want to use od (than), you would drop "in comparison" and use the comparative form of the adjective instead: Ona jest wyższa od brata (She is taller than her brother).

Why is the pronoun ona included when Polish usually drops subject pronouns?

While Polish usually drops pronouns like ona (she) because the verb makes the subject clear, they are kept for emphasis or contrast. Here, the sentence sets up a direct contrast between the brother and the girl, so ona is explicitly stated to emphasize "she (as opposed to him)."