Alec and Isabelle Lightwood are Jace's brother (and sister) from another mother. While they played a major role in City of Bones, they're a little more in the background here. The only new thing we learn about Isabelle is that she used to date a faerie boy named Meliorn. But after he guides them to the Seelie Court, and the Queen tricks them, Izzy decides never to see her faerie beau again.
Alec has a little more going on. He's still secretly dating Magnus Bane and almost comes out to his parents, but everyone stops him for some reason. He later helps Jace break out of the Malachi Configuration (that's Cassandra Clare-speak for "cage") and fights off a bunch of demons in the book's climactic battle.
The biggest revelation in City of Ashes is we get to meet Jace's other mother, his adoptive mother Maryse Lightwood, and his other brother Max Lightwood who "had always hero-worshiped [Jace] in a way that he didn't worship [Alec]" (1.1.31).
We're not sure if Max is relevant to the greater story at all yet, but Maryse single-handedly puts the whole plot of the book into motion when she basically tells Jace he's no longer welcome at the Institute. She later says she does this to protect him from the Inquisitor, but is she really protecting Jace, or does she not trust him? You be the Inquisitor and judge for yourself.