While Sirius is technically part of a binary star system, Sirius A is so much brighter than its companion, Sirius B, that we perceive it as a single, very bright star.
It's important to note that "brightest" can be interpreted in different ways. If we were to consider absolute magnitude (intrinsic brightness), then other stars in the Milky Way are actually brighter than Sirius, but they are simply further away from us.
