Non EU citizens who are family members of an EU or non EU citizen, wishing to work in Belgium are subjected to specific rules.
1. Non EU citizen family members of an EU citizen :
2. Non EU citizen family members of a non EU citizen:
A non EU family member of a Single Permit holder is authorized to work, from the moment of the reception of his/her residence document or registration certificate: the commune will add the mention « job market : unlimited » or « marché du travail : illimité » to the same document.
See chapter «Employee non EU » and « Self-Employed non EU »
More info on Brussels Economy & Employment