Rupert probably has no idea what he did to cause you to tell him he was bad - his "punishment" was after the fact. Your brother and sister shouldn't play with that whistle around him ever again, as it is probably a painful sound for him. You now need to wrk with your brother and Rupert, how old is your brother? You'll need to be very diligent over the next few days to convince him that Rupert is not to be feared, that it was an accident, and the whistle is bad around him because doggies have far superior hearing to humans, and it probably hurt Rupert's ears.