What is the ruling on leaving my device for my brother to play games that include magical elements or incantations, and should he be prevented from doing so? And what is the ruling on a game that includes a character displaying the cross symbol and increasing friends' points, and am I sinful if I avoid playing it in certain rounds?

Electronic games that undermine Islamic creed, such as those that endorse and glorify magic, are forbidden. The mere naming of a game as "magical" is not the determining factor. It is not permissible to enable your brother to play forbidden games, whether he is an adult, because it falls under the category of assisting in sin and aggression, or a minor, because it leads to him becoming accustomed to forbidden acts and corrupting his mind.