Does Apple refund prorated cost to people who cancel their personal storage plan to join family plan before end of billing period?
According to Apple:


  If a family member is already paying for their own iCloud storage plan, they can choose to switch to your plan or keep their own and still remain part of the family. When they switch to the shared family plan, they get a refund for the remainder of their personal plan. They can't keep their current plan and use the shared family plan at the same time.