In the chart, find your size, then look at the sizes directly to the left and right of your size, they're your sister sizes.
What makes them "related" is that no matter the band size, the cup letter changes, but not the volume it holds.
If you go up a band size, come down a cup size. Likewise if you go down a band size, come up a cup size. So if a bustier you'd like to buy isn't in your size, check to see if it's one of your sister sizes and it will still theoretically fit you.
Example: for a 32C,, the sister sizes are 30D and 34B