My African Grey started attacking my two sister . he has never done this before?

birds when they go through puberty and become adults may get aggressive and territorial. Make sure that he gets at least 12 hours of complete darkness and rest, if they do not get that they tend to get a bit cranky. When he gets aggressive do not yell at him, just put him back in the cage, leave him alone in the room and give him a time out for a few minutes only, no longer they have a short attention span and if you leave him alone for longer he may not understand what you are trying to communicate. after the time out get him out again and see what happens. If he gets aggressive again then repeat the discipline, eventually he will get it. Just remember that birds are like children if you want them to behave you have to be consistent

good luck

African Greys are not known for their accepting behaviours. You should cage your parrot when company comes over and place a sheet over it. This should minimize the stress caused by strangers.

African Greys are wonderful birds but it seems that they attach themselves to one person.

I'll bet you notice he is never mean to you...

when he is mean to others it's like a jealousy thing.