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When two lineages originate from the same branch on a phylogenetic tree or share a common ancestry, they are known as "sister groups" or "sister taxa." The term refers to the closest relatives in the tree, sharing a recent common ancestor from which they both diverged.
Extra: In evolutionary biology, a phylogenetic tree or evolutionary tree is a branching diagram or "tree" showing the inferred evolutionary relationships among various biological species or other entities based on similarities and differences in their physical or genetic characteristics. The term "sister groups" comes from the metaphor of the tree's branches, where each branch represents a lineage. When two lineages come off a single 'fork' on the tree, they are considered to be each other's closest relatives, much like siblings in a family. This concept is important for understanding the evolutionary history of organisms, as sister group relationships can provide insight into the characteristics of common ancestors and the sequence of evolutionary events.