OP, could D#1 reassure D#2 that there won’t be comparisons like in high school? I’ve seen many threads here in which high school students are reluctant to go to colleges where lots of their high school classmates will attend. But high school and college are such different settings that it’s seldom if ever an issue. And even more so with the size of the flagship, different majors, and the fact that both wouldn’t be in attendance at the same time.
(Note that I say this as the mother of one. But one who attended a high school so big that she once told me that if she got sick of all her friends, she could sit in the other side of the gigantic cafeteria and meet all new people.)