If you would like to know how many sandwiches will each sister get, you can calculate this using the following steps:
6 sisters ... 4 sandwiches
1 girl ... x sandwiches = ?
6 * x = 4 * 1
6 * x = 4 /6
x = 4/6
x = 2/3
The correct result would be 2/3 of a sandwich.