T26E4, while you might be right about the “iffy” school hypothesis, based on conversations I have had, in general I don’t think that the grades of a sibling currently at Princeton would impact an applicant negatively. They don’t really have time to look at that factor, and having a sibling is not enough of a push to bother. Now, if someone in the admissions office actively knows and likes/dislikes the sibling, that of course could subconsciously impact the vote of that admissions officer.