Am I sinful for buying a mobile phone for my brother that he uses for unlawful things?

If a person buys a mobile phone for another with the intention of permissible use, and does not think that it will be used for haram (unlawful purposes), then there is no sin upon him if it is used for haram, and the sin falls upon the user. Nevertheless, he should be advised and guided wisely. However, if he knows at the time of purchase that it will be used for haram, then he is sinful for assisting in sin, according to the Almighty's saying: "And do not cooperate in sin and aggression."