Is the person who gives his sister a computer sinful if he expects his younger brother to use it for music, and he has previously advised his sister not to enable anyone to use it for acts of disobedience?

The fundamental principle in such matters is Allah's (SWT) saying: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression." If it is highly probable that your action will aid in sin, then it is not permissible for you to undertake it. Therefore, if you anticipated that your younger brother would use the computer for something forbidden by Allah, such as music, and you knew that your sister would not prevent him, then you should not have connected it. If you did, then repentance and seeking forgiveness are obligatory, along with disavowing every forbidden act for which the computer is used, and advising your sister and brother, enjoining good and forbidding evil. Part of complete repentance is striving to undo what you have done by connecting the computer, if you are able. Otherwise, repeat the advice. However, if you thought that your sister would only use it for permissible things, and you stipulated to her that she should not allow anyone to use it for forbidden purposes, then there is no blame upon you. But if she does not fulfill the condition, then advising her becomes obligatory.