Answer:
Stephen's best defence is that Charlie is mentally unstable.
Step-by-step explanation:
Stephen's best defense is that his father made the new will after suffering great grief from his wife's death and was mentally unstable as of the time he wrote the new will.
He can argue that his father had a previous will where he left everything to his wife and after her death, coupled with the grief, he went to live with his daughter Marie. Living with Marie made Charlie believe he was with his wife because he had someone taking care of him like his dead wife so in his mental instability, he wrote a new will and left everything to Marie, his daughter who was taking care of him.
Stephen can also argue that he loves his father and his claims that he doesn't visit him is false because after his wife's death, he became schizophrenic and couldn't recall events accurately. He can make moves to have his new will rendered obsolete because Charlie was not mentally stable as of the time of making the new will