A sister publication generally relates to two or more publications that cluster under a related 'umbrella'. For instance, in my field, two particular sister journals cluster in a related manner. The Journal of Advanced Nursing and The Journal of Clinical Nursing are both published by Wiley, have similar impact factors and ISI rankings, have a similar past history - and past and current Editors in Chief (I.e Roger Watson) are shared.
As Dean said, sister publications are multiple publications produced by the same parent organization. I would not expect them to have the same impact factor. That is, I believe that the impact factor would be calculated independently for each journal, even though they are produced by the same publisher.