Is the wage received for helping to write ledgers pertaining to a brother's work in customs permissible?

The ruling on your work depends on the ruling on your brother's work in customs. If his work is permissible, then your work is permissible, and if it is forbidden, then your work is forbidden. Customs work is permissible if it is taken in exchange for services to the state or due to its need to spend on public interests; otherwise, it is forbidden.