There are two types of Sibling Discounts on the 1 Setup page "Sibling Discount Type" drop down for a Youth Registration Program.
Most commonly used even if just one Program is being setup. If you want to enable the Sibling discount across multiple programs, select this value for "Sibling Discount Type" and click the "Click here to link programs" link to setup the Programs you want to include.
For the "Within Offering" option only siblings registering into the same Offering will get the discount. If siblings registered into different Offerings they would not get a discount.