I don't think that Tom is going to be Sydney's brother. My Reasons:
1. That would be too weird, just a replay of a tired storyline(Nadia). I'm glad they introduced Nadia in the way they did, but I don't think they could pull it off again.
2. I think that the writers are setting up a romance between Tom and Sydney(and possible a triangle, if they bring Vaughn back). But if Vaughn doesn't come back to the show, they need a new romance.
3. THEY NEED A NEW ROMANCE!!!! I am sorry to all those S/V fans out there, but Vaughn was a wimp last season. Except for a brief interlude in the middle of S4, he always obyed Sydney's wishes. I felt like slapping him and screaming, "STAND UP FOR YOURSELF, MAN!" A guy like Tom will provide Sydney with more of an equal, someone who will challenge her. Plus I think Jack likes him more than Vaughn