Is it permissible for a sister to give her brother an intramuscular injection without necessity?

If the injection is to be given in the 'awrah (private parts), it is not permissible to do so unless there is a legitimate need, and the general rule is that a man should administer it. However, if there is no need for it, such as when he can administer the injection himself, then neither a man nor a woman should administer it. If the injection is to be given in a place other than the 'awrah, there is no harm in administering it to him.