In general, a conjunction is a word that is used to link, or put together, other words. Conjunctions can link words, phrases, and clauses.
Correlative conjunctions are pairs of words that are used to link words together.
The most important thing to remember when using correlative conjunctions is that the words, phrases, or clauses that are put together must be the same type. That means that nouns must be put together with other nouns, verbs with other verbs, adjectives with other adjectives, and so on. This point will be more fully explained following the list of frequently used correlative conjunctions. This correlative conjunction links together two words of the same type. In this case, the types of words are the nouns, sister and brother.