This sentence is a break from the main story line as John tells us some background information about the brothers of Jesus. (See: [[rc://en/ta/man/translate/writing-background]])
All of Jesus' brothers were younger than he was. This can be made explicit in the translation as long as it does not suggest that Jesus also had older brothers. Alternate translation: "his younger brothers" (See: [[rc://en/ta/man/translate/writing-explicit]])