Your post is very confusing. Can you answer these questions?
–Is your sister already at University of Maryland, or is she planning to go there in the future? IOW, older sister or younger sister?
–If she is already a student there, is she full pay (no financial aid at all)?
–Why do you think that if you go to another school that you and your sister would only pay half the cost of attendance at each school? It’s not (necessarily) true.
–Even IF it would be the case ( and that’s a very big IF) that you each pay half the COA at two different schools, how are you figuring that that would be less expensive than you BOTH attending UMD? You would go to UMD for free + your sister would be full pay = one full COA for two students.