It's not really a legacy, but an advantage.
The schools want to have "loyal" alumni families that can, hopefully in the future, give back to schools generously.
When you have multiple kids attending the same school, chances are they would give to the school more generously.
Of course, when one kid is smart in the family, the other ones should not be too bad (not just by gene, but by the family environment the kids are raised). Hence, the school will be taking a lower risk when admitting a sibling than a kid from another family.
So it will only be logical that schools give sibling applicants an advantage.
