I learned about this the hard way... My first daycare clients ever were a set of 3 siblings. She paid full price for the youngest, 10% discount of the middle and then 15% discount on the third. Yes, I know, I am shaking my head too, LoL.
After that, I decided on doing 10% off of the least expensive (older) siblings.
I have since decided that I will not offer a sibling discount, not as a rule anyway. I am not offering any less care, food, supplies, etc. The nice thing about siblings, is when they are on vacation, etc you are down 2 kids, so easier days. Same with sick days, etc. But when they do leave, it's 2 spots to fill, which can be such a pain.
I MIGHT give a discount to a long-time client, who is coming off mat leave and bringing me their newest baby. But this would be more a loyalty thing, and at my discretion completely.