You can tell if you should go for a sister size based on the following factor,
Gaps or Spilling - If you notice bust spilling while wearing a bra, you must try wearing a SISTER UP SIZE for a perfect fit. On the other hand, if you notice gaps while wearing the bra, you must opt for a SISTER DOWNSIZE.
Hook Setting - Instead of wearing the bra on the middle hook, if you find it comfortable wearing it on the last hook, you should try SISTER UPSIZE. On the other hand, if it fits perfect on the first hook, you should try opting for SISTER DOWNSIZE.