I don't eat before leaving, but my brother usually eat before leaving. I usually drink tea, but my brother always drinking coffee.
Sentence 1. Subject "I" and verb "eat" are both in first-person singular, so they both are agreed.
Noun "brother": used as a subject and verb in Present Progressive Tense "eat" used as a predicate. Subject and verb are not agreed in tense. The correct verb is "eats" as verb in Simple Present Tense third-person singular.
Sentence 2. The same with sentence 1.