A sister group or sister taxon is a systematic term from cladistics denoting the closest relatives of a given unit in a phylogenetic tree. The expression is most easily illustrated by the cladogram, where A, B, and C each represents a group:

The sister group to A is B; conversely, the sister group to B is A. Groups A and B, together with all other descendants of their last common ancestor, constitute a clade, here clade AB; the sister group to clade AB is C. The whole clade ABC will itself be rooted in (branched from) the larger tree, which offers yet more sister-group branches that are related but further removed from the leaf nodes, such as A and B. As per cladistic standards, A, B, and C may represent specimens, species, taxon-groups, etc. In cases where they represent species, the term sister species is sometimes used.

The term "sister group" is used in phylogenetic analysis; and only those groups identified in the analysis will be labelled as sister groups. An example is an analysis of birds, whose sister group is commonly cited as the crocodiles; but this is true only when dealing with extant taxa. The bird family tree is rooted in the dinosaurs, making for a number of extinct groups branching off before coming to the last common ancestor of birds and crocodiles. Thus the term sister group must be seen as a relative term, with the caveat that the sister group is the closest relative only among the groups/species/specimens included in the analysis.