Will I be sinful if I buy a phone for my brother and he uses it for music, or if I supervise a forum where YouTube is displayed and religious and non-religious clips appear, even though my intention in both cases is good?

You must advise your brother and forbid him from using his mobile phone for sinful purposes. As for YouTube, it is permissible to benefit from its useful content if temptation can be avoided; otherwise, seek another means. The rule is to cooperate in righteousness and piety, not in sin and aggression. You can transfer useful videos to your own platform to reduce the need to access YouTube and its harms. You are absolved of responsibility by alerting others to the prohibition of forbidden content and how to avoid evil.