«إِنَّمَا الْمُؤْمِنُونَ إِخْوَةٌ»
The believers are indeed brothers to one another.
Innamā is a recognized restriction construction. The statement strongly establishes the bond of brotherhood among believers; it does not claim that every believer is the biological sibling of every other.
Relative restriction must be read against the alternative assumptions addressed by the context. The social and ethical purpose is part of the intended meaning.