Is one who repairs a car stereo—that his brother uses to listen to music, and he is unable to prevent him from doing so—sinful?

You must strive to guide your brother and convince him of the necessity of abandoning sins, including listening to forbidden music, while providing him with beneficial lectures and permissible anasheed (Islamic vocal songs). If he repents from listening to forbidden things, then there is no harm in repairing the car's recorder. Otherwise, it is better not to repair it to avoid assisting in sin.