CMU does not guarantee to meet full need. Even as they define it. Run your your parents’ and sister’s financial information through CMU’s NPC on their web site as if you are NOT going to college and see how much financial aid CMU’s calculation would result. You can then see if CMU even considers your sister alone to be ELIGIBLE for financial aid according to their formula. The gap would then show clearly.
Then run the same numbers through CMU’s NPC, adding the information that you, too, are going to be going to college. That should give you what CMU’s formula is for two students in college, ideally. With zero aid.
It is then entirely up to CMU as to whether they will give your sister aid, now that there will be two college students. Knowing what I do about CMU, they may well ask what your college cost is going to be, and adjust your sister’s aid package accordingly. Since they have not given your sister a dime of aid in the past, it is entirely possible that if you get a full ride for college, they continue to offer zero aid to her.
Though you say you are not a Full Pay family , that is exactly what your family has been for the last two years. According to your, they have managed to fully pay for your sister’s college costs somehow.