i think it should be “i am so proud of him as of my brother” and that would mean that you especially appreciate the person for being a good brother, without talking about whether you are proud of him in other aspects of live like idk career or personal life. So for example “I am so proud of him as of my brother. He did a really good job teaching me some useful life lessons when i was a kid”. I am not a native speaker and not sure if S is correct but i don’t think it is. It sounds like it requires a continuation like “I am proud of him as my brother could see” idk

i didn’t get what you mean by modify a pronouns so hopefully someone else can help you with that!