The questions in the title. Just curious to hear your inputs.
While Princeton does not publish any kind of data to my knowledge regarding the number of siblings who are admitted, I am aware of quite a number of students who have siblings who also attend or who did in the past. This seems to indicate that having a sibling there (present or past) is a plus. Graduate school really shouldn’t be perceived any differently than undergrad IMO. However, I don’t know how much it actually increases the probability of admission. I’d suggest probably only slightly.
It can’t hurt but unfortunately it’s not something to rely upon at all. I know kids with good scores/resumes who did not get in even with legacy via their parents.
Not sure about how having a sibling at Princeton affects your chances, but as per this article from May 7, undergrad and grad connections are equally-weighted: http://dailyprincetonian.com/news/2015/05/legacy-status-remains-a-factor-in-admissions/