Rey being sent away by Luke really should have been the Impetus for Kylo’s turn to the dark side, not Luke’s fear of Kylo’s darkness.

Luke tells us about himself grappling with his own status as a “legend.” If Rey had been a half sister of Kylo Ren, the words of Yoda “there is another,” could have thematically connected to Luke’s struggle. IE Luke was going to be upstaged by the raw potential of the next generation, vanity, anger, etc. (over her parentage) also being a factor of why Luke sends her away.

I think force backs could be used to change who Rey’s parents are and to erase this idea of Luke as a psychopath hellbent on killing his own nephew.