First, create a discount
To make it a sibling discount, trigger the discount to add to any children added after the first.
Click trigger --> Add product to user's cart and hide from others --> set trigger conditions
Select Registration --> "Sub unit / child number is from:" enter 2 to 100 (or however many max you'd like the discount applied to --> Click "that's it, I'm finished."
You could also make a special discount for the second child only (2-2), or copy the discount to make it for children after the 2nd (3-100).