Anybody who has a sibling when growing up knows what it means to get his/her possessions taken without giving any permission, especially if they are the same sex and around the same age. This is somehow "normal" when very young.
The problem for me is that my brother is still acting in this way, even if we are both in our 30s.
We don't live together and we don't even meet very often, but for some reasons we still have a lot of possessions that are in "shared" spaces. For instance, we have a family vacation house in the mountains where we both occasionally go during the holidays. I used to keep there some clothes, some bikes and a lot of other stuff to make my holidays more pleasant.
What does my brother do then? I can give you a few examples:
Very often I find my clothes dirty or somehow ruined. For instance, once he used some of my "vacation" clothes to work in the garden and to paint and they were basically unusable after that. When confronted he shrugged it off as "well they were old and at least now there are some working clothes around here".
One year he sold some of our old bikes (which belonged to our grandfather) without even consulting me. When confronted he's comment was "well they were not really yours, were they? At least I freed some space".
One year he told me he was using my "good" bike to get around. When I went there after a few weeks he was not using it anymore. I tried to use it and discovered that was broken and needed some costly work of maintenance. I asked him about it and he said that when he used it last time it was COMPLETELY fine, perfect. Nobody else used it though.
More than once it happened that I left some tool in the apartment of my parents and found it broken after a few weeks. After a quick investigation, I discovered that he had used it multiple times.
This happens very frequently for a lot of stuff and when confronted he generally gets very aggressive, at the point of threatening physical violence if I insist on his responsibilities. He never offers to "repay" something he broke or to help in any way. He always answers that what he did was perfectly acceptable (even when mostly it isn't) or that he was not responsible (when clearly there is no other excuse).
My parents (that are still alive and in their 60s) are generally very passive about this. They somehow understand that he occasionally has this behavior but (and I agree with them here) don't want to intrude. Sometimes they offered to repay for his damages but I have always refused because I really don't think they are responsible in any way. The only reason I include them in this question is because they are the main reason I still interact with my brother and it may be relevant.
So well the question is, how should I deal with someone like him? At the moment my strategy is to just avoid him, and be more careful with my properties. I try not to leave my things around him and such, but, as you can guess, this causes my some inconveniences. Any other idea?