My nephew

Hi there! Ok so I have a question about my 5 yr old nephew. He blames my sister for everything that goes wrong even if she isn’t present. It is crazy to witness this. So for example, he will trip and blame her even tho she was not close to him. His dad picked out his clothes for him and then when my sister picked him up he said “you made me cold all day because you picked out this shirt.” It is daily and he is mean to her. She is so Sweet to him but said that he will always treat her like this and just accepts it as her fate.
Is there something she can do to help this situation do you think? Counseling for him?