Yes they are. Scourge's mother in the manga looked at a picture of a cat named Jake. And Jake was mentioned having ginger fur like Firestar. And so that was the Erin's way of giving a little hint that Firestar and Scourge are half-brothers. It's a little complicated, but on the official warriorcats forums, it's really common knowledge. Like, EVERYONE KNOWS the two are half-brothers.
