a.k.a shifting around the bra size chart (see our table below). There are two simple principles to 'sister sizing':
1. Go up a back size = go down a cup size. e.g. a 30DD becomes a 32D.
2. Go down a back size = go up a cup size. e.g. a 36DD becomes a 34E.
If you go up a back size but stay the same cup size, you'll notice extra room in the cup.
If you go down a back size but stay the same cup size, the opposite will happen, and you'll notice the cup is tighter.
Choosing to move around sizes, is usually done on comfort/ fit preference. PeBe sports bras have been sized to fit "true to size", so if you are confident of your bra size from a recent fitting - order that size.
If you aren't sure on your size, use our peBe bra size calculator here.