How legitimate is it to buy a plot of land from an elderly, impoverished woman, whose brother—her guardian—is pressuring her to sell it for half its real price?

The woman's guardian must act in her best interest, and therefore, he is not permitted to sell the land for less than its fair market value. It is not permissible for you to buy from him, knowing his misconduct, for that would be assisting him in deceiving and exploiting his sister, as Allah Almighty says: "And do not cooperate in sin and aggression." The brother must fear Allah and advise his sister for her good, as the Messenger of Allah, peace and blessings be upon him, said: "There is no servant whom Allah has entrusted with a flock, and he does not protect it with good counsel, but he will not smell the fragrance of Paradise."