الْمُؤْمِنُونَ إِخْوَةٌ
The believers are brothers.
Syntactically this is a subject–predicate relation. Semantically, brotherhood extends beyond biological siblings to a relationship of shared faith and mutual obligations, conveyed through a familiar strong human bond.
One may explain the expression through figurative brotherhood or comparison, while avoiding an unjustified categorical claim about a single fine-grained subtype. The central point is the use of a familiar relation to illuminate religious solidarity.