At the moment, the system cannot automatically add a discount for any subsequent students in the same family.  The best way to track the second, third, fourth, etc. siblings would be through an attribute, as they are added to the system.  There are two ways to add this 'discount':

Create a separate Product & Billing Item  (BB202965) for each discounted rate and apply it to the records.

OR

Add the charge to the record as it is and then add a credit (BB379496) for the discounted amount to apply to the charge.

A credit can be added to a group of records(BB203392) if it's the same discounted amount.
If attributes have been created, this group can easily be found through a query.