"My brothers read books."
Translation:मेरे भाई किताबें पढ़ते हैं।
भाई is its own plural. You can make out that it is brothers (plural) because it has been conjugated with पढ़ते हैं.
However, you are right that the sentence 'मेरे भाई किताबें पढ़ते हैं' can also mean 'My brother reads books' if the brother is elder and you are using the 'respectful' form (when the singular भाई would also be conjugated with पढ़ते हैं). You have to figure out if the sentence is referring to 'brother' or 'brothers' from context.