What should I do with my wife who rarely prays, and am I allowed to prevent her from communicating with her brother who mistreats me, or should I divorce her?

Neglecting prayer is one of the greatest abominations. You must advise your wife with gentleness and firmness, and compel her to pray. If she refuses, then use legitimate disciplinary measures such as boycotting her in bed, and then striking her. If she does not respond, divorce her and seek a woman of faith. As for her brother's kinship, do not prevent her from it unless he is corrupting her against you. If he is harming you, then you should boycott him and strive to improve the relationship with him.