Short answer
A family member applying later generally needs to show real access to enough money for themselves and any family members included in the application.
Family members applying for a Student visa later generally need to show they have real access to enough money for their planned stay in Australia.
The amount may include living costs for the student and family members, school fees for school-age children, and travel costs for the student and family members.
Where a 12-month calculation is used, it is based on the person's planned stay in Australia.