In modern standard Spanish, you normally use a possessive adjective like mi directly before the noun:
You do not usually say el mi hermano in standard modern Spanish. That sounds archaic or dialectal.
So:
Spanish often does not use an indefinite article where English does.
English says:
In Spanish, after ser, many nouns and adjectives appear without un/una when describing what someone is.
With adjectives like miope, no article is needed:
Because mi is the possessive adjective used before a noun:
Mío is a possessive pronoun or stressed possessive form, usually used without the noun or for emphasis:
So in Mi hermano es miope, you need mi, not mío.
It matches both; the difference is just regional English vocabulary.
So if the meaning has already been shown as either one, that is fine.
Mi hermano es miope is the most neutral and standard word order.
Spanish can sometimes change word order for emphasis or style:
For a learner, the normal pattern is:
Yes. Mi hermano es miope is fully natural and standard in Spain.
In everyday conversation, speakers might also say things like:
But Mi hermano es miope is grammatically correct, natural, and standard.