Spanish uses ser and estar for different kinds of to be.
Here, trabajador describes a more permanent or characteristic quality: your brother is a hardworking person. For that, Spanish uses ser:
Using estar trabajador would sound unnatural in standard Spanish. If you want to talk about how someone is behaving in a specific moment, Spanish would normally use a different expression, such as:
In this sentence, trabajador most naturally means hardworking.
So:
This is a very common pattern in Spanish: a word that can also be a noun is used as an adjective to describe someone.
If you want to say My brother is a worker, Spanish would more naturally say:
But in many cases, Spanish prefers a more specific noun, such as a profession:
Because here trabajador is being used like an adjective, not as a noun.
That small difference changes the meaning.
So if there is no article, learners should usually understand it as a description of the person’s character.
Because this is a normal Spanish sentence pattern:
So:
This is similar to English in structure:
Spanish adjectives often come after the noun when they directly modify it, and after ser they commonly appear as predicate adjectives, as in this sentence.
Yes, but it means something slightly different.
Sometimes both are true, but they are not exactly the same.
Someone might work a lot because they have many hours, while ser trabajador means being hardworking by nature or attitude.