Sister sizing for bras is used when you need to make slight adjustments to your band fit without changing the overall cup volume. This is achieved by going up in band size and down in cup size, or down in band size and up in cup size!
Using the chart, you can find your sister size moving diagonally up or down.
Move Up Diagonally: Maintain cup depth and increase band size
Move Down Diagonally: To maintain cup depth and decrease band size