I did at first. Then I realized that I am not feeding one of the siblings any less, or giving them any less time/attention, or doing less in terms of crafts/activities. Some may say that siblings are a benefit, because it's TWO or more filled spots. But it's also a risk, because if it doesn't work out, that is TWO or more spots to fill.
I no longer offer sibling discounts, but I have 2 (soon to be 3) sets of siblings in care.
What is the norm in your area? If you are looking for ways to draw in families, this could be a good way to do so, too.