The Senate doesn't seem to follow the same rules as the others. They could not possibly afford to maintain all those troops that have from a single city, for one. They seem to just have new family members pop into existance for another. So, I don't really find it suprising that they would hang around with no family members. It's probably coded into the game that the Senate faction is eliminated when Rome is lost. I know in my last game, I caught Rome with the massive Senate army elsewhere in Italia (had been chasing a bandit army all the way down into Bruti territory near Tarentum), and when I captured Rome, the entire army vanished. It didn't turn rebel, it just disbanded completely.