Is it permissible for me to hack into my brother's device and delete movies and pornographic material from it, knowing that he does not accept advice, does not pray, and watches forbidden things?

You should advise your brother in the best possible manner to abandon the sins he commits, such as abandoning prayer, listening to music, and watching forbidden films. If you are certain that he possesses content on his device that is not permissible according to Islamic law, and you are able to delete it without his knowledge, then there is no harm in doing so. This is because it falls under the category of changing an evil with one's hand, and this will ward off a great evil from him and from those children whose corruption is feared. It must be noted, however, that this is to be done only to the extent necessary, and it is not permissible to go to extremes in it or to spy on him.