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.
Often some sort of coupling pin.
Oh! And if she's into puzzles, look at Islands of Insight. First person with puzzle solving as the main mechanic, but also some exploration and a bit of mystery to it.
I'd consider what kind of game she finds fun and compelling--that's more likely to engage her to become familiar with the mouse/keyboard combo.
That being said, Minecraft on peaceful mode would fit and is generally approachable. Stardew Valley is pretty relaxed as well, though I don't remember whether it uses WASD specifically.
Remember the old ThinkPad idiom: "If the cracks are just plastic, it's still fantastic!"
I'm really sorry to hear that. Sometimes life puts us in a place where we need our family, even if they're awful.