How to declutter when it’s not your stuff

TLDR: I am living with family who are fine living in super cluttered homes - I want to have a clear out but don’t know how to approach it.

I am currently living between 2 homes, my brothers flat and my dads flat. Our mum died last year (who I used to live with) and we’ve just sold her house so all her stuff has been shoehorned into any space we could find in their two places and we desperately need to go through it but it’s too emotional for anyone to deal with still.

To make matters worse, both my dad and my brother are massively hoarders themselves. My brother is a collector, collecting sports memorabilia and trading cards and Lego and special edition dvds and any other crap you can think of. I have a room in his flat which I keep as tidy and minimalist as possible, which keeps me sane, but the rest of the flat is a free for all full of his stuff. He gets irritated if I move any of it or ask to keep communal areas more clear/neutral.

My dad has books, papers, files, you name it, from the last 60+ years that he doesn’t care to go through. I don’t have my own room so I have even less space for my own things. Half of the stuff in his flat he can’t even get to because it is unreachable behind piles of crap. It’s driving me insane (not least because the place is dusty and dirty due to this hoarding attitude and it gives me allergies) but he is genuinely OCD and doesn’t let us touch anything (even cook or clean) never mind declutter without getting very stressed, angry, and upset.

Obviously, I hope this is a temporary set up until I can move out and have my own place and live how I like. I also understand it’s their space and they can live how they like and are entitled to have as many belongings as they want. However, I can’t help thinking we’d all be better off with a good spring clean and clear out of all this stuff sitting around. I really hate this situation and I don’t invite people round because the flat is such a state and I honestly just want to blitz it myself, take it all the tip, and let them deal with it later. I have culled my stuff as much as possible so I barely have anything left but my family’s stuff is a no-go so there’s not much more I can do now. Help!!