@tigertiger66, my D just went through the application process and will start Princeton in September as a member of the class of '20. I just pulled out a printed copy of her CA as well as the letter that Dean Rapelye sent to me (and all alumni) in late November after she submitted her SCEA app.
On the CA (I believe the main section) there is a question about siblings and education level. My S is still in HS so I don’t know if the dropdown menu choice of “some college” will bring up a place to enter that a sibling goes to Princeton. Also, I believe the Princeton supplement directly asks if the applicant has relatives who work at/attended Princeton.
In the letter to alumni there is a question - How can my student indicate our family’s connection with Princeton? The relevant part of the answer reads “In addition, if other members of your family attended Princeton, as undergraduate or graduate students, we welcome this information in one of two ways. Applicants had the opportunity to list other relatives on the Common Application, or they mail email this information of the Office of Admissions at [email address].”
I absolutely think that they do look at the relevant section … because how else would they have complied the list they used to send the letter?
Hope this helps. Good luck.