Siblings may share one or both parents. Hence they are either full or half siblings. A DNA analysis can determine which is the case.
A DNA test can be conducted to determine if persons share the same biological mother and father. A full siblingship exists between persons who share the same biological mother and father. If they only share their mother or father they are considered as half siblings.
A DNA siblings test can also be an indirect way to establish family relations when the putative father is not available for testing.
Of course, also a joint biological mother can be determined between the participants.