Sibling discount for 1 time Events or class

To support our large families in the community we offer a discount for recital. Ex: $200 child 1 and $175 child 2. Our company hosts so many families with multiple children, we can not offer sibling discounts on tuition. It’s too big a financial loss. So we want to offer this special for the recital. We prefer not to have to manage a credit. And hope for a simple option for one time events and classes.