Though your sister was eligible for financial aid at SOME colleges, for whatever reason, CMU decided not to give her any. Your parents and she decided to pay CMU’s full price and somehow did so for two years.
It’s entirely up to CMU Ad to how much they will give her this year with a sibling going to college in addition to her going this third year. Upperclassmen generally get their awards after incoming freshmen do, but if she is going to continue at CMU, she’s at their mercy as to what they decide to give her in aid. As I said earlier, they are highly likely to take your costs and awards into consideration. However, that doesn’t mean they will come up with grant money— I’ve known past CMU kids offered loans to meet gaps in aid. They are not a particularly generous school.
You should have a talk with your parents about what they are will and able to pay. They likely feel on the hook with CMU since your sister is half finished there. How much are they truly willing to spend for you? if they really cannot pay much more than what they are for your sister, the answer is pretty clear where you should go