The rules tell us that you can use Sustained Attack on a Close Combat attack. Now Close Combat is a two-way street - the defender gets to counter-attack. Can the defender use Sustained Attack on their Close Combat response? Clearly they haven't moved, as it is not their activation...

You may only do a Sustained attack on your activation, so, no the Defender would not get the bonus, but could, if they survive, then do a sustained attack back...