Hello jkjk84m2,

A sibling volume is a volume is created from a snapshot. Your volume is the parent and the snapshot is the sibling. 

Please let us know if you have any other questions.

Thanks

Just to be clear the term sibling volume is not always used. In many instances it may be called a View Volume. Look at page 9 of the Dell Storage Center OS 7.0 Data Reduction with Deduplication and Compression
http://en.community.dell.com/techcenter/extras/m/white_papers/20442456

Data reduction and view volumes
When data reduction is enabled on an SC Series volume, due to inheritance it is also enabled on the view volumes created from the volume snapshots. In other words, view volumes share data reduction settings from the parent (or source) volume. Likewise, when disabling data reduction on a parent volume, it is also disabled on any dependent view volumes.