Hello Lucael: I see this is your first post here on PC. Welcome to Psych Central.

In your title you asked if what you are experiencing can add to / trigger your PTSD. I'm not a mental health professional. So I can't give you an authoritative answer. But, from a lay-person's perspective, I would say yes absolutely what you're being exposed to can add to / trigger your PTSD. Plus you really shouldn't have to be exposed to what your brother & his gf are doing anyway. Unfortunately, given the circumstances you're living in, I don't know as there is much you can do about it other than try to find a way to find somewhere else to live.

You mentioned having selective mutism. There is a sub-forum, here on PC, dedicated to the subject of Social Anxiety & Selective Mutism. Here's a link just in case you haven't already found it:

https://psychcentralforums.com/socia...ective-mutism/

I hope you find PC to be of benefit.