In general, yes, the EFC gets cut by 40-50% for the second student. In other words, the expected family contribution is half of each. Run the NPC for BC and change from 1 to 2 in college and see what it calculates.
Does your sister attend a private school, or lower cost public? Does she receive merit aid at that school? Also, note that each school figures its own version of NPC. For example, Harvard has one calculation, Cornell, another, and BC a third formula…