There is no "solution" to this. And this is really your wife's problem now. If she wants a relationship with her family (and if you want one), you'll have to accept the family dysfunction as is.

Since that seems unacceptable to you, I would advise you to limit any contact severely or eliminate it. No more cabin talk. No more "helping" with the cabin. No buying outright or partially of the cabin. The cabin is out of the question. Assume that your BIL will get it when MIL dies. It will fall into disrepair and that's that.

What I read in your post is you are actually looking for a way to change THEM, but as you know this isn't going to happen. Move on with your own family life and stop giving those people space in your head.