Is it permissible to repair computers for siblings who watch movies and listen to music on them, if not repairing them would not prevent them from having them repaired elsewhere, and this is done to protect the father's money?

You must advise your brothers regarding the seriousness of their disobedience to Allah. If they do not heed your advice, then gently and kindly explain to your father that he is participating in their disobedience if he aids them with his money in listening to and watching evil things. If he does not comply, then you have fulfilled your duty. Do not help them fix the computer as long as you know they use it for things that displease Allah, based on His saying: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression. And fear Allah; indeed, Allah is severe in penalty." If your father gives them money to fix it, then the responsibility lies with him and them, and no sin will befall you as long as you have forbidden the evil to the best of your ability.