About 2-3 years ago the BC admissions office responded to this question. They said it made a difference, but they offered no specifics.
I have a different view.
BC admissions has to say that legacy can help. They are a business that depends on alumni donating to the school. If they said anything other than “yes”, it would hurt those donations.
Read though past years of legacy discussions in this BC section of CC. You will see some very angry alumni who have given their $100-$500 per year only to have their child not get accepted.
I can imagine if you’ve given a lot of money to the school over the years, it can help.
Some schools are known for legacy. BC is not one of them. If it tries to build a well rounded highly talented diverse incoming freshman class, then influencing decisions just because an applicant has family legacy isn’t going to help with that goal.