Family Issues Support Group
Family issues is a huge range that go from minor conflict to major misbehaviour and even abuse on the part of individual members of the family occur. Whether you feel you are in a dysfunctional family or you're dealing with a very specific issue, this is the place to talk about it and find others who might be going through the same thing.
I do think that you should have a conversation about him, but it should be about decision making and respect.
As for the stuff, forget about it. It doesn't sound like it's got much value, and his sister IS moving out...