FAFSA asks how many students are in college and doesn’t ask how much Student A is paying or how much Student B is paying. It’s purely how many are in college. Having a sibling in college i the same year basically cuts the FAFSA EFC in half. However, all the federal EFC determines is federal grants (Pell) and usually whether the student loan is subsidized. If your sister’s EFC is $60k, cutting it in half brings it to $30k, and that pretty much means no federal benefits for her.
You are at UMD and if your EFC was cut from $30k to $15k, you’d still get few federal benefits. MAYBE a subsidize loan.
CMU is a CSS school so your sister had to fill out the CSS and that changes everything. The CSS does ask about the sib’s COA, and CMU may consider that cost (doesn’t have to, but can).
Do not go to another school. Worry about your own education. B/K is a great opportunity, not only for the tuition but for the experience.