This was my favorite thread last year on Duke vs Princeton: http://talk.collegeconfidential.com/duke-university/1657701-duke-ed-vs-princeton-scea-p1.html
I believe the OP in this thread decided to apply early to Duke and got in.
I do think sibling is a plus for an applicant. Does your younger son have similar qualifications as your older son? Plenty of kids get into ivies with strong school leadership, but what I have observed (from reading lots of cc) is that state or national level out of school accomplishments are often (but not always) the “wow” factor that makes a candidate more compelling for EA at HYPSM.
At my son’s school (private) the GC advise students to apply ED to Duke if 1) it is a top choice 2) they have no substantial hooks 3) they have a profile like your son’s, as chances are better in ED versus RD.