What should I do about my doubt that my brother has not performed ghusl after major ritual impurity (janabah), and am I sinful if I do not inform him, and will his prayer not be accepted in this case?

A Muslim must do what Allah has commanded him to do. The presumption for actions performed by their rightful doers is validity and the absence of doubt regarding their correctness. This is based on the Hadith of Aisha (may Allah be pleased with her) concerning eating meat about which it is not known whether the name of Allah was mentioned over it or not. Therefore, one must abandon doubt regarding a brother's ghusl (ritual bath) from major impurity and build upon the presumption, which is the validity of the ghusl. If the brother is ignorant of some religious rulings, he can be taught through indirect methods, such as sharing posts or gifting booklets about the rulings of ghusl, its requirements, and its manner.