I still don't totally understand why "é" is necessary here.
In the copula, if you have a definite phrase, you need the corresponding pronoun. a deartháir is definite (it's talking about a specific brother), so you need the é before it.
Can I not also say 'Tá a deartháir ina dlíodóir'? It's marked as incorrect, but it's ok, isn't it?