Swearing....what to do??

hi im looking for some advice regarding my nephew who is on the autistic spectrum, i also have two sons myself with autism and am trying to help my sister with some ideas regarding this swearing problem...we disagree as to wether this is a part of his condition or wether he does it because he can...this only happens at home, my sister says its when he is angry or upset, although he doesnt do it anywhere else when hes upset or angry...now when i say swearing it is directed at his parents, he calls my sister a b**ch and a c**t and he calls his dad every name under the sun, however it seems to have become a battle between his parents, his dad is angry at being abused like this so he gets angry and starts shouting and arguing with him, but i think that is the frustration of constantly having my sister tell him he cant help it, its part of his condition. She says she attended a one day course last year just after he was diagnosed where she was told to ignore it, but it is so bad and getting worse and he has mild autism and some learning disabilities, his condition is in no way moderate or severe. Mysons conditions are similar and yes they do swear when angry and one of them swears a lot playing computer games but it is different when thier son is being abusive directly to them and calling them names everytime he cant do something, or hes asked to do something he doesnt want to do. Can you please let me know what you think, but more importantly if you think this behaviour should be challenged or not, i feel a little as if my sister is putting every behaviour down to his condition and she is afraid to tell him off and i think now he is 11 but he will soon be 15 and i cant imagine how bad it will be then, also suggestions on what to do about it if anyone has any, thankyou