Our current fee billing software allows us to manage sibling discounts by automatically managing sibling links. It knows therefore to allocate each second and subsequent younger sibling with a 10% discount. When a sibling joins or leaves it would recalculate the discounts due.
An automated process would save a lot of time when producing the invoices as we would not need to have to manually go through all 1000+ pupils to establish which siblings should have a sibling discount applied.