"He is my brother."
Translation:Lui è mio fratello.
There are a few things wrong with "lui i miei fratello".
First, there is no verb. You would need to say "lui è i miei fratello."
Second, you have the possessive in the plural, but the noun is singular. You would need to say "lui è il mio fratello" in the singular or "loro sono i miei fratelli" in the plural.
Third, singular family members don't take the definite article as part of the possessive phrase. So the correct way to say "He is my brother" is "Lui è mio fratello."