Is a brother considered responsible for his adult sister and sinful if she does not respond to his advice regarding her clothing and places she frequents, or does the responsibility lie with the parents since they are the guardians?

The father is primarily responsible for his daughter, but the brother must offer advice and rectify evil in a manner appropriate to his situation. If the brother fulfills his duty of clarifying, enjoining good, and forbidding evil, then he bears no sin if his sister commits transgressions, nor does he fall under the description of a cuckold (dayyuth). Patience, gentleness, and forbearance are recommended when enjoining good and forbidding evil, as it has been transmitted that one who enjoins and forbids must be learned, gentle, and forbearing.