The black bishop is standing up on the ropes, about to do a sweet jump
owenfromcanada
The general meaning is the appearance of truth or validity.
But I usually use it to describe something that is "believable" even if the underlying premise is not. So a fantasy story that pays close attention to detail and is highly consistent might be described as having versimilitude. On the other hand, a story where the characters make out-of-character choices might be lacking versimilitude, even if there are no overtly "fictional" elements to the story.
That's usually how I've heard it used, not sure if it's the "main" usage though.
That all sucks. To answer your question: decide whether you really have to deal with them forever. In this case, unless I'm missing something significant, it sounds like you could cut your brother and his family out of your life. It sounds like their contributions are exclusively negative, and after going behind you and your mother's backs with the land thing, you'd be perfectly justified to cut them out.
In short: don't deal with them.
"Wisdom pursues you, but you've managed to outrun it."
or
"Wow, it must have taken all three of your brain cells to come up with that."