إِنَّمَا الْمُؤْمِنُونَ إِخْوَةٌ
The believers are indeed brothers to one another.
“Innamā” is a standard restrictive construction. “Al-muʾminūn” is a nominative subject and “ikhwatun” its nominative predicate. In this structure the “mā” prevents “inna” from operating normally, so “al-muʾminūn” is not an accusative ism inna.
The emphasis is on solidarity and brotherhood among believers. The contextual restriction does not erase every other social identity; rhetorical scope must be understood from the purpose of the address.