They first see Edward when he is too far away even to be recognised for a gentleman, so if the servant had been riding near him, they would have distinguished two riding "figures". If the servant was a couple of hundred yards behind, however, and the road was winding (it is hilly country), so that they both popped in and out of view as they approached, it would explain why they might not notice him until he appears at Edward's side at just the right moment.

It did strike me when thinking about this incident that the sisters must be nearly blind if they can't even make out that it isn't Willoughby until he gets to a mere 30 yards (i.e. 30 good paces) away. The "popping in and out of view" scenario, combined with the presence of hedgerows and other head-height vegetation, would help explain this.