I’m not convinced that Duke is in any measurable sense “more fun and less stressful” than Princeton. The only kid I know who went to Princeton was horribly unhappy and in fact should have graduated several years ago, but hasn’t. I don’t think he’s typical though. He sounds like a kid who would be happy in either place. I think they have a fairly similar vibe.
Whether or not there is a legacy bump from being a sibling is something you will have to ask the admissions office. It varies by school, there is no Ivy rule about how to decide whether someone should be a legacy. Some only count parent who were undergrads, some count parents who were grad students, some count grandparents, and some are interested in siblings.