Is my brother sinful if he unintentionally touched my خال's wife, and am I sinful because I saw him and did not warn him?

Your brother committed a sin by touching a woman who is unlawful for him, and he must repent and seek forgiveness. It is obligatory for you to denounce the evil and advise your brother, based on the Prophet's (peace be upon him) saying: "Religion is sincere advice," and his saying: "Whoever among you sees an evil, let him change it with his hand; if he cannot, then with his tongue; if he cannot, then with his heart—and that is the weakest of faith." If you do not do what you are able to do to denounce it, without a valid excuse, you will be sinful.