Sister sizing is a technique used by bra fitters when they just can't find the right bra for you with your natural size. Sister sizing is not as ideal as the perfect fitting bra, but you may try it if you are struggling to get a bra that you like in the size that you need.
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.
Take your normal size bra (let's use 32F as our example) the sister sizes are 30FF and 34E.
In the chart, find your size, then look at the sizes directly to the left and right of your size - those are your sister sizes.