A dyad is a group of two things or two people. If you have a dyad of brothers, there are two of them.
While you can use the noun dyad to mean “pair,” “couple,” or “duo,” it’s much more common to find the word used in a technical way. Sociologists, for example, use dyad to refer to social groups that consist of only two people, like a brother-sister dyad or a mother-child dyad. The word stems from the Greek dyas, “the number two,” or “a group of two,” from duo, “two.”