I think your expectations are wrong. Perhaps at 8 you still might tolerate your mom hovering around but at 12? It's part of puberty to grow apart from your parents, it's just biology. You can be glad he is comfortable even bringing his friends to your home so you can meet them and talk to them at least a little bit.
The only a bit weird thing is that they get the whole house/apartment and you're confined to your room. I'm not sure about your living situation but it should be the other way around, they should be in his room.