Since CMU does not guarantee to meet full need for all students, there is absolutely no guarantee she will get a nickel more in need based aid than she gets now.
My older kid went to a private university about the same cost as CMU. When his little sibling was a college freshman, he got a rousing $250 additional merit aid. Let’s just say…that didn’t come close to paying much for his college costs…but every penny counts.
Our family also did not quality for need based aid with one in college.
If your sister (or parents) had wanted to save money, perhaps one of the colleges that offered need based aid to your sister would have been a better choice.
You have the chance to go to undergrad for FREE. That is a huge gift to both you and your parents. UMD is a terrific school. I’m having trouble figuring out why you don’t understand this.