It could be normal for him to just suddenly turn into a barker at his age as he is at the age where he is a teenager and shelties are known to bark a lot. It may be just a phase or it may stick. It could also be partly due to your sister for agging him on & you for not getting him more used to "strange" objects.
Training will not help much unless everyone is willing to help (including your sister) but when he does bark you could sternly say NO & then ignore him, or you could try to pre-occypy him w/ a toy or game or treat before the object comes into view or hearing range (which may be hard to do). Also by getting him out more & used to these objects & situatuions may help. Giving him more excersize may wear him down a bit so he is not "looking" for things to bark at. Good luck