Is it obligatory to inform my brother that I saw his nakedness without his knowledge for my repentance to be accepted, or is it not obligatory to inform him for fear of severing or affecting our brotherhood?

Allah has forbidden looking at private parts, and you have committed a sin in this regard. Therefore, you must repent immediately by abandoning this act, regretting it, and resolving never to return to it. These are the conditions of repentance. It is not a condition to inform him of this act for your repentance to be accepted; in fact, do not do so, especially if you know that informing him might lead to the end of the brotherhood between you.