Only the player being targetted can defend himself with a Family Influence card. None of the other players can help him out, either.
Not how I learned the game - and there is nothing in my rules about this.
There is one passage about what happens if two players try to play a counter card - and is says that only the first one takes effect. This could just be refering to Finger, but there is not mentioned anything to exclude Family Influence or Mob Power. Even if you would rearly help another player, it could be good if the attacker has lots of mobsters left and you and third player only have one.