For my college, no official stats of sibling legacies have ever been published in the +25 yrs as an alum volunteer. However, I assume that if the applicant were on the fence, the fact that another sib is already enrolled could either be a plus or minus – here’s what I mean. Let’s say the older sib was admitted from a previously unknown school, i.e. the admit factors were a little hazy but still enough to offer admit. Older sib goes to PTon and is failing miserably.
Younger sib’s transcript/scores/rest of file – looks the same. PTon may not be as willing to take a 2nd chance – because indications are that the HS preparation isn’t enough, despite high grades. The opposite is true too. Maybe older sib is performing beyond all expectations. This will then give confidence to the veracity of younger sib’s HS achievements
Is it fair? Not really. But sometimes the committee members are looking for any sort of confirmatory info to decide tough cases.