Ben Solo-Organa breaks bad and becomes Kylo Ren, at the behest of a still-somewhat-mysterious villain.
As Leia laments, he just had too much Vader in him, which must be a persistent concern for any and all Skywalker offspring.
Which would include Rey (or whatever her name was when she was just another Solo-Organa kid).
Luke, whose training inadvertently caused Kylo Ren to join the Dark Side,
realizes what a potential danger Rey is to everyone around her.
He takes her from her parents something we see in a very, very brief flashback as young Rey is left on Jakku
Then uses the Force to mind-wipe everybody he can get to, to erase their knowledge of Rey's existence.
Which makes sense to me in a way
Han/Leia's kid
Luke hides here and mind wipes to protect her
yeah that's what I thought
which is why he seemed to look so sad when she turned up
But like "the force guided her here"
"For still yet another, it explains why Kylo Ren the only character who couldn't have been mind-wiped seems so fascinated by the person he calls "the girl."
He must know, on some level, who she is. and brother vs sister is more dramatic than fight between cousins