“Cousin Race” is a term used to mean two races that are capable of breeding and producing a child. The two races will almost always have very similar body structures. The child of a successful pregnancy between such a pairing is universally sterile.
Large groups of races that are all considered cousin to one another are called clades. Not all races within a clade are capable of successful pregnancies, or at least the number of successful pregnancies is so low as to be considered unsubstantiated rumors. Larger clades might be organized with sub-clades, with members of the sub-clade being known for much higher chance of successful inter racial pregnancies. The largest generally accepted clade is the Homid clade, which includes Humans, Orcs, Dwarves, vulcans, and many others.
The term “cousin race” is used within the tradelands and is not always true biologically. Some races with outward similarities are biologically incompatible. An example would be the supposed cousin races of the Tau, Protoss, Vedalken, and Salarians, none of which are able to interbreed (and some sexual pairings, especially with the Salarians, that would be physiologically impossible). This misinterpretation stems from a lack of universal understanding of genetic inheritance, uneducated speculation, and rumor.
That said, there are some races that are genetically incompatible that are linked through other means that are considered cousin races anyway. The Myr and Warforged represent an unusual example of two races that are dependent on the other. The Flamekin and Gensai are believed related in some way, but due to their magical nature this relation cannot be properly expressed in any biological sense. The relationship between the subraces of mermaids, some subraces of Naga, and the Orochi is profoundly unclear even to the members of said races.