Like us, Orion was a dog lover. He had two loyal canine companions, the big dog and the little dog, who followed him, even in death.
Presumably he was stuck up in the sky so that Artemis (Zeus's daughter, who loved him) would stop grieving over his death, since she would still be able to see him at night The two dogs were sent to keep him company.
Since Sirius, the brightest star in the sky (two stars actually--It's a binary system) is in the Canis Major, or big dog constellation, it is called the dog star.
Their sacrifice appears to have been halfway successful . Although summers are still hot, the werewolves, at least, are gone.
This is the only reference I could find that links Sirius specifically with werewolves.