Sister group

A sister group or sister taxon is a phylogenetic term denoting the closest relatives of another given unit in an evolutionary tree. The expression is most easily illustrated by a cladogram:  Taxon A  Taxon B  Taxon C A sister group or sister taxon is a phylogenetic term denoting the closest relatives of another given unit in an evolutionary tree. The expression is most easily illustrated by a cladogram: 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 most recent common ancestor, form the clade AB. The sister group to clade AB is C.