All members of the applicant's family unit must be declared in their application, whether or not they intend to become applicants for a student visa.
If the applicant is the holder of a student visa and a member of their family unit was not included in the application form, they will not be eligible for grant of a student visa as a member of the applicant's family unit, unless they give evidence that they became such a member after the decision to grant the student visa was made. For example, the applicant got married or had a child.