If you imagine your sternum was broke into 2, it was wired together when surgery was finished, with several pieces of very tough wire. This is why you can cough and sneeze straight after surgery without your chest flying open.
It takes about 6 weeks for the bone to make a good start in knitting together and after 3 months this will be substantial. Full recovery takes around 6 to 9 months. So when you say the sternum is 'moving', I doubt if this is the 2 halves moving against each other. If this was the case, then it could never join back together and would require re-wiring.
You did not mention if you have had a chest x-ray or ct done lately. I have read alot about complications with the sternum having too much movement after surgery. Unfortunatelly sometimes the sternum wirers move or break which can cause the movement. If the friction continues it could cause an infection. I would suggest for you to talk to the physician that did you surgery . I hope things work out well for you. I also hope this helps.