First of all, UNC is a very good university. I have only met a few graduates from there (I live way to the north and east of you) but they were all excellent. One UNC graduate that I knew was one of my best friends and one of the strongest students in the program when we were both graduate students at Stanford. Graduate admissions at top schools do know how strong UNC is.
Congratulations on already having been admitted to this excellent university (with the note that you clearly have earned this admission through your own hard work over the last 4 years).
At the Yale, Princeton level you are not all that far away from being an average applicant. You know what the acceptance rates are like for these schools. You are typical of the students that they accept, but you are also typical of most of the students that they reject.
One thing that would worry me is whether or not other schools might be affordable if they consider your sibling as also being a university student, but then might become unaffordable after your sibling graduates, which suggests that it might look affordable for year one but not stay affordable for years 2 through 4. I am not however familiar with how need based financial aid works. Hopefully someone who is more knowledgeable about need based financial aid can clear up my concern one way or another.