Taxonomic sisters (equonyms) are expressions that are in a paradigmatic relationship with each other; that is, they are interchangeable within a sentence and incompatible with each other. They are at the same level in a taxonomy.
Example: Terrier – Greyhound – Chihuahua – German shepherd – Dalmatian
These terms are all hyponyms of ‘Dog’ and so are sisters in that taxonomy.
They are interchangeable: “I just got a new dog; he’s a ________________.”
Any of these words would fit in the blank, but only one can make sense; a dog cannot be more than one breed (otherwise it is a completely new breed).