I’m wondering how much my legacy status will improve my chances if I apply ED
Generally speaking, Section C7 of the Common Data Set for a school will tell you how much emphasis that school places on “alumni/ae relations,” if any.
Legacy can help you quite a bit if your family members have stayed involved with NU (e.g. helping with alum interviews, fundraisers, teaching). If the alum has NEVER participated in any way, then it might not help much, if at all.
I hope your family belongs in the active group. Good luck!