Legal Question in Real Estate Law in Washington

If the property were not uninhabitable, I would advise you to have him sign a month to month lease for $1 a month. That makes him your tenant, and gives you the power to terminate the tenancy at any time (with at least 20 days' notice).

However, now, tenant or guest, without the time to research it here, you are likely in violation of several health and safety laws, given your brother is living there without proper sanitation in a dilapidated building. I would be more worried about that than "homesteading." I think what you mean when you write "homesteading" is adverse possession. In order for someone to claim another's land by squatting on it or otherwise using it, the person claiming title by adverse possession has the burden to demonstrate that they occupied the land in an open and notorious manner, that they had actual, uninterrupted, and exclusive use and possession of the land, and that such use and possession was hostile to the true owner's title, for a period of at least 10 years.

The ten year rule is why I am less concerned about adverse possession than health and safety liabilities created by the dilapidated condition of the cabin. If I were you, and I was still serious about allowing my brother to live there, I would get him a camper trailer to live in while he sorts out the cabin and gets it up to snuff. I would sign a lease agreement with him providing either $1 a month in rent + his labor in exchange for living there. That way, if anything ever goes wrong, you can evict him and be secure.