To keep things short, we know Demacia is very anti magic in all their thinking and life style. Would Garen have to kill his sister if he found out she has the power to wield magic? Or would Lux be sentenced to death if she was found out to be a magic user? I ask because the updated demacia lore paints Garen as a very black/white character when it comes to good vs evil and has no in between space for anything.
last ive read on the matter is, that mages in demacia either get lobotomized and/or live outside the city in slums.
i doubt hed actually do anything to harm his sister though. hed be more likely to review his black/white moral code or try to save her in some way by only getting her banished, like he did with taric.
if im not mistaken, taking the crown of stone means for many people to not actually climbing the mountain, but it means the same as banishment from the city and theyre usually starting over somewhere else.
taric was one of the rare exceptions who actually attempted the climb to redeem himself, which garen probably never expected of him.
on top of that, garens and lux most recent lore is very early on in their story. we already know, that lux is going to be trained and work for a group of mages inside demacia. in time hell probably find proof of what he already knew, and its more likely to change his views on mages to a more tolerant approach than end in conflict between them.
