My older brother has pornography in his possession and I have known about it for quite some time. I don’t know if he knows that I know about it. (We were both exposed to it when we were young boys. His possession is a stumbling block to me so I strive to avoid his room.) Am I participating in grave sin by not talking to him about it?
By your question, it sounds as if you and your brother both still live at home. If that is the case, then the ones to whom you should talk about your brother’s stash of pornography are your parents. It is their responsibility to instruct and correct him in this matter.