I came from a upper-middle class family. We were wealthy enough that qualifying for financial aid at an ivy was basically impossible. We were poor enough (with 3 of us in college at once) that it wasn't possible.
This was particularly problematic because my dad (a military doctor) had only been out of the military for a couple of years. Meaning the salary he we were being pegged at for financial aid was something he had only been earning for 2 years or so. He mad significantly less in the military.
Point being: For some folks, in some situations.. the cost is well beyond reach.
I don't know when you graduated, but a few of the very top schools with huge endowments (e.g., Harvard) have started expanding the range of their financial aid program to cover more middle-class families, like (it sounds) yours. This is a pretty new phenomena though, in the past ~5 years.
The Ivys and liberal arts places I know take multiple children going to school at the same time well into account. I'm sorry for the trouble, but I'm not sure if you didn't look into it, or if you are talking about a situation well into the past. That's just not the case anymore.
EDIT: Hell, look at the comment below; the net price calculator (http://npc.fas.harvard.edu/) even has a drop-down for number of children in college.
The issue with CMU is that, essentially, they don't have much money. Their endowment is much smaller than most of the schools they compete with. In terms of the "top" CS schools, they're probably a bit of an outlier.
That's one of the reasons I ended up not going to CMU. Well, that and the fact that I didn't want to trade the Bay Area for Pittsburgh :P.
This was particularly problematic because my dad (a military doctor) had only been out of the military for a couple of years. Meaning the salary he we were being pegged at for financial aid was something he had only been earning for 2 years or so. He mad significantly less in the military.
Point being: For some folks, in some situations.. the cost is well beyond reach.