how much pull does being a legacy at an ivy give you to help in getting in?
From data I've seen, it's about a 30% boost in chances (unless the parents make a huge donation to the school or something like that). DebaterMAX also has a point that that may partially be because legacy students typically come from wealthier households with better access to things like prep resources and classes—those students just have a leg up in general—but there is definitely a sizable boost beyond just that.
It is sizable but I have little knowledge how much is just because the students get extra assistance from their parents as in prep classes. But yes legacies have a boost to ivy caliber schools. Here are some links I found via googling.
Hope these help!